How "selective" they are has always been determined by the applications on file.

Every application gets an initial score by the computer. What gets you points and how many it gets you is a nuclear access codes type guarded secret. But it is a safe assumption that more:

PIC time, Multi time, 121 time, ratings , etc are worth more positive points
DUIs, checkride busts, etc are worth more negative points.

Once your "score" is determined you are put in the stack of all applicants. Then when UA wants to interview 100 folks, the 100 highest scoring applications on file are sent an invite. When new applications come in, yours will get bumped down the list by somebody with a higher score. When folks with better scores are invited to an interview you will be that much closer to the top the next time invites are sent out. About all you can do is continue to update your app info regularly so that whatever experience gains you have had can be appropriately added to your score.

With all the recent events around hiring at some carriers and downsizing at others, it is safe to say that UA is getting some very experienced folks applying at this time. Couple that with a pause in hiring and suddenly we are much more selective. We have Captains from other non-regional airlines applying right now and if it stays DL and UA hiring and AA not for the next several months, then I would imagine it will get more difficult to get an interview in that timeframe unless you have significant PIC time.

On the good news front, a vast majority of folks that apply at us are also applying at DL and AA (and others) so as one of us hires somebody they are then not really in front of you on the other list anymore. Ultimately there are a crap ton of retirements coming up in the next 5-10 years so movement is still going to be rapid at all the big carriers, it needs to be just to maintain their current size. Imagine what is needed to grow!!