I don't really have the answer for your question, but I can attest that this also happened to me recently, a mini MR from STL-HKG.

Outbound: 4 segments---> got the first 3 boarding pass, agent seem to try to print out the last one but couldn't figured out (and can't seem to understand why). Got the last one at DTW with no problem from some random GA.

Inbound: 5 segments---> again, got the first 3 boarding pass, the last two are printed at a connecting airport (however, this particular agent mentioned something about 6 hrs out so she can't print the boarding pass, true or false, I don't know).

Based on this, I would say maybe cause you have >3 segments. @:-)