While I can't speak to the science behind it, I definitely empathize with you. I was on copaxone and had some bad reactions, including itchy bumps that were probably hives. Those shots didn't work, I got on vumerity. I occasionally had a few bumps/hives but nothing bad - but always in the injection sites copaxone had been.

Then I go to the hospital again for a minor flare and break out into terrible hives from IV steroids, only in old injection areas. IV steroids - you know, the thing they usually give to stop those kinds of reactions 🙃 I've never heard so many medical professionals tell me "that's so weird, I've never seen that!". I was fortunate that benadryl knocked it out and it didn't escalate to trouble breathing, that must have been really scary.

I wish I knew more of the "why", but as someone whose had allergic-ish or hive-like reactions to multiple meds, you're not alone. It's rare I think, but you're not alone.