A synch roll, or sync roll is a phenomenon that occurs when the synchronization between the recording head and the video encoded on the tape is lost and gradually resynchronized. It usually occurs when there is a gap, drop out, or misalignment of the videotape control track that is used to tell the VCR or VTR where the beginning of each frame begins. When the control track suddenly reappears, the head may already be in the middle of reading a frame of video, in which case the speed adjusts to synchronize with the video on the tape, causing a vertical "rolling" or "scrolling" of the picture on the television.
References
Synch roll Wikipedia(Text) CC BY-SA