Bradley's left foot gave way on the sodden turf, literally slipped underneath him and clipped his other heel, that's all there is to it.
He didn't decelerate purposefully, he wasn't deceived by a feint from Diallo, the turf broke away when he planted his foot.
If you don't believe me feel free to personally review the footage here:
https://youtu.be/7ciUjT_TAlA?t=82It's a shame because if it weren't for that piece of misfortune he would likely have prevented a goal. He defended the situation perfectly, aligned himself side on, matched Garnacho's speed progressively, controlled how the distance closed between them and forced the pass into Diallo. Even after the slip he recovered admirably and forced Diallo to take on a contested low value shot - it just happened to squeeze into the miniscule gap between Bradley, Kelleher and post. Fine Margins.
The criticism of him for both this situation and his performance in general has been completely unwarranted.
Perpetually exposed to a 2 v 1 overload in general play after his introduction, and then thematically exposed to a 2 v 1 for the decisive goal, the criticism should be directed elsewhere.