Did you use raw pineapple juice or was it heat treated in some way? Raw juice would contain some enzymes that break down proteins and the resultant fragments could increase foaming. Just a random thought though, I'd be curious if cooking it reduces the effect.
Only used pineapple myself as freshly juiced, a tiny bit for bottling kombucha, instead of sugar for carbonation.
Otherwise, could be the fruit contains some unfermentable poly-sugars that increase foaming.