I think the top rib can be mobilized a number of ways - the PTs always did the technique while I was sitting up and them behind pressing / pushing down, the chiro does more of a tweak thing when I am sitting or laying.

Thinking back on it, mine did pop up a few times, from certain uses, but I knew what it was right away and chiro was able to get it back down easily.

I suppose if TrPs are in the scalenes or SCM it would keep the muscles from totally releasing also.

chiro used some pretty firm thumb pressure/tension when he worked on my neck muscles and that was quite awhile before the top rib mob was tried by the PT.
I was off work by then and really focusing on doing the home stretches, TrP, rest and relax, heating pad, posture work...