What may cause a perfboard to bend?
I have noticed that perfboard does all sorts of bad things: corrosion, bending, peeling, etc. It even bends when stacked neatly in a pile and is not in use. 

So I think the best answer to your question is "it just does", or perhaps "it uses cheaper, less stable laminate methods, and all laminates are at risk of bending." (Plywood for example often warps when one side is exposed to air and the other is not.)

My recollection is that it is made of FR-2 (synthetic resin) and not FR-4 (bonded epoxy) like commercial circuit boards, and is not as stable a base.