Somehow, there is a difference in the thumbnailing of one individual movie file and multiple movie files. With the latter, some movie files will fail to get a thumbnail or ThumbsPlus will even crash. I have long ago stopped looking for the cause, and Cerious could never reproduce the problem.
Nowadays, when I want to thumbnail multiple movie files, I simply select them and press INS ('tag file'). That causes them to be added to the Tagged Images gallery, which in turn makes ThumbsPlus thumbnail them in 'individual mode', without crashing or otherwise failing (well, usually). Afterwards, I press Ctrl-INS to remove the tags. So, my suggestion is that you use this as a work-around.