Simple, Tommy is more trusting of others and has shown that in the past and also wanting to help others. Hes the same guy at the beginning of the first game where there is an outbreak and he wanted Joel to pick up people alongsidd the road not knowing what was going on, whose infected, what the infection was or anything.
Which seems at odds when you're a patrol guard in an apocalyptic setting guarding a town and keep logs of bandits and infected you've dealt with. Also why wouldn't it cross Tommy's mind people might be after Joel after hearing his story at beginning of the game?
Tommy can't be the same guy as in the beginning of part 1, he and Joel joined hunters and killed innocent people to rob till he eventually got sick of it. Such life experiences should change anyone. He's not that thug anymore but he also knows how rotten people can be in this world.
Sure I can see them be friendly towards strangers after it's clear they mean no harm or aren't a threat. But they didn't know what that armed group was there for so acting cautious around them would seem the natural thing to do at first.
Joel didnt givr up names, Tommy did, and also Abby gave up her name first not an alias or anything. So in this world and part 2 it seems there was/is a level of trust when dealing with strangers who are in need of aide and confide in each other.
Joel shouts Tommy's name several times in the room with the cable cars where you fight off the horde with Abby.
Also they weren't "sureounded" originally, everyone was chill in that room until the names came to light. Everyone was acting pretty friendly towards each other before everything went left and hostile. They shared information on each other, who they were, where they were going, hell even Mel gave her name before everything went left.
Tommy stays on the side but Joel places himself almost in the centre where he's partially surrounded.
Tommy and Joel surviving that long(and as former hunters) should know how kindness can be fatal in this world and that some level trust has to be earned first. (Giving out information you can't verify doesn't make you trustworthy.)
If Joel and Tommy werent Joel and Tommy it seems there wouldnt have been an altercation in the first place.
You can still keep all the events that lead the brothers to Abby's group(as contrived as that it) but from there changes can be made to make the brothers not make these mistakes and still progress the story as it did in the game. In post #516 I write about it.