I don’t know anyone who has spent a significant time at a major/legacy pax carrier then made the jump to UPS.
I wouldn’t say it is the most common thing. However, you do see some of it. The reasons I have heard vary from looking for greater stability (worries about displacement/furloughs significantly affecting income), local to a UPS domicile, and a lot of guys who were actually furloughed from their legacy came to UPS and chose to never go back (more common among guys who came here in the 90s and 2000s).
It is not a risk free decision, but you will likely be better off financially at UPS and living in domicile is tough to beat. Sure, first year will combine bad pay with minimal seniority. However, we are growing and supposedly the CEO wants to order additional jets to those already on the books. Guys are wiped out right now because we have been swamped with business (especially in Asia) and international flying is just a rolling train wreck between hotel lockdowns and constant trip revisions. I’ve been here over three years and am at roughly 70% system seniority and around 49% in my fleet/seat. The top half of the seniority list will be retired between now and 2029, so it’s very likely that 4-5 years will be first available upgrade (probably ONT 75, A300, SDF 75 in that order), which would beat when you would choose to upgrade at Delta. Tough decision to make, but congrats on putting yourself in the position to have the choice.