Seemingly unwinnable odds of Lee driving through a herd...
Their reactions and sidelong glances at each other behind Lee's back.
They knew his clock was about out of time...how much of a chance did he really have?
They said early in their intro into the game that they believe being on their own was safer....this was a good time to get back to that, despite the hard choice of leaving Clementine out there...
Just saying it could explain a lot.