You're about a month late with that advice. I think a lot of CO2 gets trapped under the fruitpack and pushes the whole thing higher than you expect. It might be worse with the pineapple because it's very fibrous.
I think you've hit the nail on the head. With my banana wine, when it was at its most vigorous, you could actually see the airgap underneath the cap as it was being pushed up the neck of the demijohn!