Yeah same thing happened to me. When I thought about it after, it made perfect sense to send him with them. I have no idea why I sent Thane.
Well, I imagine escorting them back means avoiding battles as much as possible and both Thane and Mordin have backgrounds in stealth (Thane as an assassin, Mordin as a member of the special task group).
My logic was
Vents - Tali, because she's a mechanical genius and I just didn't trust Legion.
Second team - Garrus, because he's my bro and he has experience leading a team of people, and he doesn't have to worry about being betrayed this time around. Just sent Garrus twice, because the only other choices were Miranda and Himuro and I just didn't believe my teammates would be very happy about following Cerberus officers.
Biotic - Jack, since they make such a huge deal about how she's maybe the most powerful biotic alive.
Escort - Mordin because he was in the salarian special task group.