I'd guess (not knowing that spot very well) that your answer may be implicit in this--the bus queues on the approach to the next junction probably get too long (quite a few bus routes use that stretch), quite possibly because they only get a green phase every two phases compared to the York Road/Lambeth Palace Road alignment (if that's the case).
The queue is two lanes of all kinds of traffic, not just buses. There's no bus lane at that point.
All other kinds of vehicles, except buses*, stop downstream of the crossing until they can get all the way across it, leaving it clear for use by pedestrians.
I don't know why that spot is so bad. I've not really noticed buses obstructing crossings regularly in other parts of London.
It's nothing to do with the roadworks - they've always behaved like that. Now the crossing is wider and the railings have been removed it's slightly harder for bus drivers to completely block the crossing, but they do try.
They could solve the problem by buying buses with doors on both sides.
*And a few white vans, but not that many.