Why are audio and video out of sync

I've been uploading videos to Motionbox recently (.mov files from my Canon digital camera, which I've just trimmed in Picasa) without any trouble. Then, all of a sudden recently, they upload with audio and video out of sync. I've changed nothing on my end, and the videos look fine on my computer. What's going on? Incidentally, these are simple videos of only one to two minutes in length and not terribly complicated. Thanks for your help!