Mexicana
I was a non-rev pax on an American flight from Cancun that landed on 4R at ORD 40 minutes prior to Mexicana. At the time the WX was still nice, with calm winds, but thunderstorms were approaching from the Northwest. On my way home from the airport I drove (northwest bound) through torrential rain for 20 minutes. My guess is that Mexicana landed into the approaching WX. I am based in ORD and have experienced many WX systems approaching... ATC will keep you going until you tell them no... I guess Mexicana maybe thought they could beat the WX... Just a guess.