There was some theorycrafting that it related to the person who received the buff for killing the last fodder mob, but I don't think anything was confirmed. It's such a rare thing to observe, it's equally possible it's just a bug and it doesn't work as intended, but they haven't figured that out yet.
Having been to about 80 Mireu fights at this point I have to assume that the awakening buff has little to do with it. Prior to the spike flail nerf I had seen known bots get the buff then die to a flail in a few seconds. I’ve seen trusts pets etc take the buff. It doesn’t matter who gets the buff. Thing to keep in mind is that once the proc phase begins melee attacks no longer generate TP so if that phase doesn’t end fast weapon skills quickly become exhausted. The exception is tp feed from Selh’teus trusts and any regain status effects including the awaken buff. It is entirely possible that the proc is a specific weapon skill or ability that occasionally rolls a rarely used job specific skill. It’s unlikely because the occasions I have seen it bug out on have actually been pops during peak hours, if this were the case you would expect statistically to see it happen during those midnight pops that even on Asura sometimes time out but most reports seem to show it reaching this phase in under 30 minutes which is only possible under extreme overcrowding. On average this phase lasts under 20 seconds