When can I make assumptions in scientific papers?
It depends on what you mean by "is it OK".  If you write "The behavior X might be explained by...", it's certainly OK in the sense that nobody will be offended or automatically reject your paper.  You are acknowledging some uncertainty, so there's nothing to object to (while there would be if you claimed greater certainty than your work justified).  On the other hand, speculating about possible explanations is not as good as giving a compelling explanation.  It's reasonable if this is not the main point of your paper, or if the speculative explanation is awfully clever.  However, if the main point of your paper comes down to speculation that you just haven't investigated carefully enough to resolve, then it will probably not impress readers.  You could end up with a paper that's not objectionable but still isn't very good.