The company I work for is a global staffing agency, though I work in the in house IT department, I know all about staffing/interviewing/hiring procedures.
They are not allowed to ask you why you left your last job. The only legal information they are allowed to find out is the dates of employement and if your last employer would rehire you.
If you want to offer up a reason why you left, that's something different. It is always best to say that you felt you were no longer growing in that position so that is why YOU are seeking other employment where you can grow professionally.