PNRs are generated by the reservation system, not the airline. UA uses Galileo.

PNRs are reused fairly quickly after they get closed, but PNRs can be open for a very short period of time up to 330 days. So, they don't close in alphabetical order, so I doubt they get reused in alphabetical order.

In other words, I think it was pretty much a freak accident that your PNRs bore any resemblance to one another.

I'll be interested to know the specifics myself.