Why doesn't slidedown sometimes gracefully move the content below it?

I have 2 buttons on a page with a hidden div underneath each. Each use the same jquery code to slideDown() the hidden object directly underneath. Both 'work' technically as in the o开发者_如何学JAVAutcome is the same, but the transition is very different.

The problem is that 1 slides all the content underneath the object nice and smooth like, while the other slides down the hidden stuff over the top of the content underneath - THEN once the slide is complete, all the content underneath snaps below it.

What would cause this kind of behaviour?