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Satellites don't just spontanously burst into 100 pieces.

Well...

There are at least three possibilities that occur to me, and two of them probably aren't done by Russia intentionally.

One is that they tested it as a target for some kind of anti-satellite weapon. It was decomissioned and probably expendable, so that'd be consistent with targets of past anti-satellite weapon tests. Russia has been talking about anti-satellite weapons and is not happy about us providing satellite reconaissance data to Ukraine. US intelligence also believes that Russia has been considering deployment of a nuclear anti-satellite weapon.

https://www.nbcnews.com/news/world/pentagon-official-warns-russian-anti-satellite-nuclear-weapon-devastat-rcna150314

A senior Defense Department official told lawmakers Wednesday that Russia is developing an “indiscriminate” anti-satellite nuclear device that would pose a threat to all satellites operated by countries and companies around the world.

"The concept that we are concerned about is Russia developing and — if we are unable to convince them otherwise — to ultimately fly a nuclear weapon in space which will be an indiscriminate weapon” that would not distinguish among military, civilian or commercial satellites, John Plumb, the assistant secretary of defense for space policy, said at a House Armed Services subcommittee hearing.

He said the threat was “not imminent” but that the Pentagon and the “entire” Biden administration were concerned about the program.

This isn't that -- that's in earlier stages and we'd know immediately if something like that were used -- but I suppose it's probably a fair bet that anti-satellite stuff is being discussed in Moscow. That'd be on Moscow, if they did that.

The second is that it got hit by some kind of debris too small for us to detect. If we don't know about it, the Russians probably don't either, and probably couldn't avoid it.

https://orbitaldebris.jsc.nasa.gov/measurements/radar.html

NASA's main source of data for debris in the size range of approximately 5 mm to 30 cm is the Haystack Ultrawideband Satellite Imaging Radar (HUSIR). The HUSIR radar, operated by the Massachusetts Institute of Technology’s Lincoln Laboratory, has been collecting orbital debris data for the ODPO since 1990 under an agreement with the U.S. Department of Defense. HUSIR statistically samples the debris population by "staring" at selected pointing angles and detecting debris that fly through its field-of-view.

The data are used to characterize the debris population by size, altitude, and inclination. From these measurements, scientists have concluded that there are approximately 500,000 debris fragments in orbit with sizes down to one centimeter. The NASA ODPO also collects data from the Haystack Auxiliary Radar (HAX) located next to the main HUSIR antenna. Although HAX is less sensitive than HUSIR, it operates at a different wavelength (1.8 cm for HAX versus 3 cm for HUSIR) and has a wider field-of-view.

Since 1990, the Goldstone Orbital Debris Radar has collected orbital debris data for debris as small as about 2 mm in LEO for the NASA ODPO. It is located in the Goldstone Deep Space Communications Complex in the Mojave Desert near Barstow, California and is operated by NASA’s Jet Propulsion Laboratory. The Goldstone Orbital Debris Radar is an extremely sensitive sensor capable of detecting a 3-mm metallic sphere at 1000 km, which makes it an incredibly useful tool in the characterization of the sub-centimeter-sized debris population.

Even with all that, my guess is that there's probably debris up there that can cause a lot of damage. The example above is small, but also a metallic sphere. I'd bet that there are some materials that are a lot more transparent to the radar that they're using.

Low Earth Orbit objects are moving at a pretty good clip:

https://www.space.com/low-earth-orbit

In very simple terms, low Earth orbit (LEO) is exactly what it sounds like: An orbit around the Earth with an altitude that lies towards the lower end of the range of possible orbits. This is around 1,200 miles (2,000 kilometers) or less. The majority of satellites are to be found in LEO, as is the International Space Station (ISS).

In order to remain in this orbit, a satellite has to travel at around 17,500 miles per hour (7.8 kilometers per second), at which speed it takes around 90 minutes to complete an orbit of the planet. 

The most common handgun round is 9mm Parabellum.

https://en.wikipedia.org/wiki/9%C3%9719mm_Parabellum

7.45 g at 360 m/s for one type of ammo, about 4.6% as fast.

So something that weighs 0.34 grams will have the same energy as a 9mm round.

A paperclip weighs maybe 1 gram. So something in LEO a third the weight of a paperclip will hit as hard as a bullet from a Glock.

It could also be a micrometeor not in Earth orbit coming in from outer space. I don't know if we can detect those. Those could be moving a lot faster (and hence could be even smaller to cause a given amount of damage).

A third possibility is that something on the satellite exploded. It's got maneuvering fuel with oxidizer...I'd guess that there are probably ways for that to blow up. If there's something that has a lot of kinetic energy, that could fail. Flywheel failures can be pretty exciting in terms of shrapnel going everywhere, and if they use gyros to do orientation, it might be possible for one of those to shatter:

https://en.wikipedia.org/wiki/Reaction_wheel

A reaction wheel (RW) is used primarily by spacecraft for three-axis attitude control, and does not require rockets or external applicators of torque. They provide a high pointing accuracy,[1]: 362  and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.

https://en.wikipedia.org/wiki/Flywheel_energy_storage

High performance flywheels can explode, killing bystanders with high-speed fragments.