My take on the Bradley sub...
Just prior to it, we'd switched to 3 at the back, with (L-R) Forsyth, Nelson and Cashin. That put Cashin on the wrong side, and I'm not sure Nelson is comfortable in the middle of the three. In order to get round pegs/round holes in the defence, he swaps Forsyth for Bradley (who is more happy in the middle of a 3), which also allowed Cashin and Nelson to swap to their preferred sides.
It struck me as a defensive tidy up after a formation change.