Its a behavior that i beleive is a result of some changes in patch 1.3.
Ive seen it often myself, and i bleive its a result of 1 or 2 things:
1.) destroyed convoy leader. This was a bug before, im pretty sure it was fixed though.
but most likely
2.) Lost contact time. See
here for details.
I think after so long, the idle escort in question will rejoin the convoy if it doesnt get too far away from him. Before patch 1.3, i used to alter the lost contact time in TM, after patch 1.3, i left it at its default.