Okay, seems weird, really, I believe.. here are my in game reasons:

After the turn call, this is probably the bottom of my range (folding QQ-99,8x, A5s,A4s are twopairs), however, what about KQs,KJs? Do we want to call with those hands OTT? Is the reg ever betting a Kx or bluffing with that sizing against 3plyrs? (with one big whale).
I was also thinking that AK,KK,AA likely 4bets preflop in his shoes and A4s,A5s might be a bit loose calls pre(maybe not actually), but still given his sizing, it looks like AQ,AJ,ATs,A9s,A7s,A6s. Okay, we might add some 88, A8s,A5s,A4s (lets say 66% of them) = 6combos. His Ax one pair region contains 24 combos. We can easily see that our raise become profitably once he starts folding AJ and worse.
That said, even if our combo might not be the best choice and we potentially might hold KQs,KJs (not sure about QJs,JTs,QTs, seems like a turn fold or raise) I believe this spot can be an autoprofit raise against the pool (they are pretty aware of the fact that river raises are just notoriously underbluffed by other regs