I was thinking same as Mikertroid, reckon he is trying to crash land on the other side of the river but the speed is so low the plane stalls, left wing drops and it comes down on its side in the river.
Can’t see any reason why he would try to land whilst banking, or try to land in a small river he is heading across at 45 degrees, neither of those are going to end well. The only reason would be to save lives on the ground but I think from what I watched on the BBC yesterday, the far bank was not built up. (watching again, there is a green patch where he was headed but not much chance of success)