Its a keirin. They are fighting for position, elbows, shoulders and a bit of leaning/nudging are pretty much expected.
A keirin where no one touched anybody would be pretty dull.
Cueff tries to edge Riessen out, touches (within the rules), Riessen moves over/out a bit faster than Cueff is expecting and she then ends up leaning the wrong way (leaning the wrong way on the banking, at speed is a surefire way to end up in casualty) luckily (?) Riessen is in the way, so she doesn’t hit the deck, shes riding pretty much on the outer edge of the tyre on a curved banking (so to even go straight she’d have had to turn the bars a far way.) shes also got her weight totally off centre (i know you MTB gods could correct at a moments notice, but she’s only a world class track rider).
So they end up crabbing across the track, until it flattens out and she’s got a bit more grip/better balance to correct. Then she hits the deck anyway.