In two separate instances Tuesday, stocks plummeted sharply for a brief period before returning to normal. At around roughly 10:18 a.m. Eastern, 88 stocks fell or rose by 1% or more. Eric Hunsader, founder of Nanex LLC, pointed out the changes on his Twitter feed. Hunsader said in an interview that the stocks that rose were being added to the MSCI index, and the ones that fell were being taken off that market index. ”It could be someone they had their own basket, and they were all interested in getting it all done and didn’t care about the price,” he said. But the market saw another mini–flash crash at around 2:45 p.m. Eastern, when roughly 40 stocks moved abruptly for roughly a second. Hunsader also noted that the stock of Medivation Inc. MDVN, +0.43% plunged twice in the same day. Hunsader said the second round was not as severe, both in the magnitude of the moves and the time involved. Eric Garcia