Hamilton was directly called into the pits by the Mercedes strategy team to switch onto slicks, which triggered a wave of first stops for the pack hovering around the edge of the points from the back of the grid.
As for why they all pitted to change tyres…
Because they weren't in the points anyway, so even if the gamble backfired, they had nothing to lose.
After the stewards noticed that the cars at the back were all diving into the pits, they finally did something sensible for once and enabled DRS even though the track wasn't fully dry yet.
If race control had opened it earlier, Qin Miao might've had a bit of a headache.
But race control only opened DRS on lap 17.
And at that point there were only 7 laps left in the race, and Verstappen's intermediates were already pretty worn.
