3/6 AK confused

[Praetor]

Effective 100BB

4 handed, I raise to 21 with AK, bu calls and SB calls.

BU is main villain 25/11/ 4.29 river agg over 1.5k hands. 3 overall. SB is 32/18/2.81

Flop is AJ9r. SB checks, I bet. BU calls and SB calls

Turn is a 10. SB checks, I check, BU checks

River is a 2. SB bets 75, I call and BU raises to 222. SB folds? I?

I feel like this is almost never a bluff but its really confusing