One of our 4 kitties started doing the same a while ago, always around 10-11am. In my experience as a cat dad of over 30 years it boils down to one of those things:

• Food/Hunger/Water: Maybe his current food/water/feeding location isn't to his liking (even if he liked it before), or the feeding time is inconvenient for him
• Attention/Space/Power shifts: Did any of the persons living in the household change up their routine? do any other pets/activities currently hog your attention? Did another cat start taking over a sleeping spot after you started your walks? Also, walking your cat MIGHT (thats cat specific) put stress on him because his now much larger territory cannot be defended while he's stuck at home; not all cats are cut out for walks even if they show no issues during a walk and many are happier just staying indoors
• Toilets: Not necessarily (only) cleanliness, but location also in respect to feeding / water locations, and probable medical issues regarding defecation/urination (one of our boys had bladder stones a few years back - he was quite vocal after toilet visits)

In our case it was caused by one or both of the 2 things: a bit of hunger around 10am (while all other cats in the household are asleep), and one of the other cats lately started hogging my wife's attention and cuddling a lot more with her, taking time away from HIS precious headscritches-time. We fixed it giving a small amount of food at 10am (just a few bites), before he starts his behaviour, and intentionally giving him a weekend of bonus attention which shifted time back to him over during normal times too. It's very cat - and situationspecific; he is definitely our most human-oriented cat, so i'm pretty sure the attention shifts were the main culprit.

Don't be afraid to try different things, but don't change too many of them at the same time and give changes a day or two to sink in. If your cat cannot get something he likes, always offer an alternative to him! I hope you find out whats bugging your cat!