Mine suddenly started doing this about a month ago. To add to what Minchkin posted, no amount of bolt tightening helped me because my seat looseness also appears to be caused by this light yellow plastic insert the seat bolt passes through (it's part of the seat height adjustment mechanism). Very annoying to be reminded about it every time I stop or start.
-come to a stoplight- seat blips forward. -light turns green- seat blips backwards.
There was a thread recently about this issue and several people posted that they also have this problem. It makes me wonder if it is systemic to our generation of Focus. I also worry about what impact it has on the safety of the driver if they were in a wreck.