First, what made up a loose passive player?  He plays a very wide range of hand, and he does not get his money in unless he hit a big hand.  With UTG open limp and call a raise, I would like to put him on a mediocre hand, like small A, may be even suited, very small pair, or some mid low suited connector.

With the given actions, I really don't believe UTG had any A, unless he really like to extremely slow play his hand, which is very stupid in this level (2NL).  So, what made him shove on the river?  With his range, J-8, 8-6 made it, and it matches the style of a passive player, or if he had a busted 7-6/8-7, he really over played his hand.  Now, you need to ask yourself, How he played his draw before? Did he play that strong or weak?  How he played his split bottom pair? Did he lead out or he just check it all the way? I would personally put him on some kind of 7 that does not connect to the board, like a K-7.  

Now, for SB, he was either any 2 board way or pair of 8s

Whether it is +EV move for you, it all come down to the fact that, do you believe UTG made his straight for less than 46% of the time, since that that pot odd you had been offered(call $2.43 to will $2.91, 1.2 to 1).  Even if you may had the edge here, since it was so small, I would suggest you decided based on the fact that, whether you will go on tilt and kill the session if you happen to lose the hand instead of what his hole cards were.

a couple more, with UTG checked twice with the As, I don't think he had any board way pair neither, since he might be afraid of someone, especially SB had an A, while SB also checked the A two times up front, it UTG had any big pair, he should be confident enough to bet on the turn.

With such passive action in both previous streets, and showing sudden strength on the river, I would tends to believe he had the nut, instead of a stone cold bluff

lets post your own analysis first, that's should be the most interest bit.... result is a minor thing

well well well... you just got unlucky and owned by his extreme passiveness.... with that 2nd A on the turn, he really should bet, since the chance of someone had another A that will out kick him and not betting on the flop is pretty slime....