Typical end-month session' dip at the close... they held 'em propped all day, then funds front-run other funds unloading their window dressings.
Happens often enough to be a pattern: my only two trades today went 0pts ER and then +3pts ER right into the close. We've seen enough of these last minute dips to expect them more often than not... especially end-quarter and index rebalance events.