THIS! I thought maybe it was me, or I was going mad or something. Glad it's happening to someone else

For now i'm going top presume it's:
1) A bug
2) a new and AWESOME way for arc missiles to proc that blizz forgot to put into the patchnotes

Although IIRC there was some mention that there were plans to up the rate at which arc missiles procs, possibly they upped it a bit too much.

are you proccing off ball of flame and thinking its just streams of missles?

No. I proc arcane missiles off of flame orb all the time as a quick mana regen tactic in between evocates as needed. What I'm talking about here is proccing arcane missiles off...something...but I haven't figured out quite what.

I cast arcane blast a few times, the cost is getting high and I'm not in a burn phase so I decide to use the arcane missiles that procced on my second arcane blast. I'm in the process of casting my last arcane blast and when the cast bar moves into the red so I can queue up the next spell I hit arcane missiles. The arcane blast goes off, I start to cast arcane missiles, and before the first missile actually lands, I've procced arcane missiles again and I can cast it immediately after the current string of missiles if I want.

What I can't tell is whether these arcane missiles are really proccing off of the arcane missiles, or if they are proccing off the arcane blast. Ordinarily, if the arcane missiles procced off the arcane blast it would just overwrite the older arcane missiles and restart the timer. But possibly, because I've queued up the arcane missiles from the earlier proc, this newer proc doesn't overwrite but just renews after the first.

I know this may sound very confusing, it's somewhat difficult to describe.