Assuming they are both regs so they know each other...
its a draw heavy flop so.
1) SB I think it has a Ahi or a low K for doing this, the turn bet/call looks like a not that strong value/cbet, and the river donk looks like I have some value and dont want to give a free showdown to a draw. (edit: also a draw like JT, J9, J9 spades or not)
2) Since BU didn't capped preflop, and given the board, his range on the turn has a lot more draws (flush and straight) and weak made hands (pocketpairs) than Kings or Queens, even if a turn raise could be for value against draws, this is also good spot to bluff against the wide range SB should have to 3bet a BU steal.
3) I think they both know and realise their wide ranges and that SB has some kind of a read for doing the donk on the river, something like "BU doesn't bluff enough on the river" or "doesnt make too thin value bets" cause BU might have bet something like 6s7s to make a a JTs, 9Ts fold.