Brian put out in a recent blog his answer to a question he was asked regarding when the team began rushing the student section and asked readers for responses and recollections.
After significant wins, (1997 win over OSU, leaving Michigan undefeated in the regular season and headed to the Rose Bowl,
and the last-second Manningham catch to beat PSU, for example) the fans rushed the field. I don't ever remember the team rushing the stands, even as a one-off kind of event.
The team began rushing the student section on a regular basis during the first year of the RR era, and only after a win, of course. I remember at the time the feeling of it was the team's response for the fans being "all in." Since the RR wins were not at the level of significance as mentioned above (perhaps only the comeback win against Wisky), the fans would not be rushing the field, so the team rushed the fans. It has been occurring ever since--and might I add at every home game thus far during Hoke's reign.