It's not random yet, its the same pairings every time the matchmaking lobby restarts.
If I remember correctly:
1. City Hall vs. Sanctuary
2. Wall Street vs. Impact
3. Downed Bird vs. Parking Deck
4. Lighthouse vs. Skyline
5. Terminal vs. Statue
6. Pier 17 vs. Evac Zone
I'm pretty sure that is correct. I know the first 4 pairings are, however I could be wrong on the last two there. Basically, a lobby has to stay open without a host drop or something restarting the lobby for four rounds to be able to get to Skyline. I agree its super frustrating. I love the map to do Air Stomps, really the only level I do it. It used to be the first in the rotation, now its so late and I play with large groups that usually people on the other team rage quit fairly often, 50% of the time they are host and we never get that far. Also, I love both Lighthouse and Skyline, Lighthouse I consistently go 20+ kills, usually with only 1-2 deaths, definitely my best map, and I hate having to pick between the two.
Long story short, I can't wait until the randomization system goes online. Cry-Adam mentioned something about it after they shuffled the pairings last time, hopefully will be included in the forthcoming patch.