I had no problems with rejections at first but then several of my clips taken using a tripod were rejected for camera movement.  There was no camera movement and those clips were accepted on all the other sites I upload to.  It looks to me that there is a big problem with footage reviews at the moment and SS haven't responded to complaints in their forum for weeks.  Here is one of several threads about this.

http://submit.shutterstock.com/forum/viewtopic.php?t=54555

Perhaps the reviewers are just rejecting clips as it takes a long time to download them to review?  I have never had a problem with SS on the stills side and I expect the same standards on the footage side.  Until I see something has been done to sort this out, I will stop uploading footage to them.