I received everything else before the 36-slot bag finally dropped on my main (first time I’d seen it on my account too) yesterday. I haven’t seen it drop for any of my alts yet; will try to keep an eye out for this.
Assuming this wasn’t an attempt to trade between factions, it’s just the system being very temperamental and unfortunately nothing new (as can be exemplified by my old thread regarding the DF pre-launch event).
The portal just spawned in Valdrakken while I was typing, so I’m currently on an alt for the “first attempt of the day” (just in case).
Welp, was just Wild Cards again. Trying an LFG for Durotar next. Hooray, bag dropped for my hunter. 
Edit: I did the portal event for the first time on my priest alt and the bag dropped. Not sure if lucky or the drop rate’s been changed, but I guess I’ll find out when I do my warrior alt next.
Edit #2: Participated in the Valdrakken portal event on my warrior alt (it was her first ever), and the bag dropped yet again. Well now either I’m outrageously lucky, or the drop rate really was changed.
Edit #3: Final edit. I had the bag drop for my druid alt (and yet again, first attempt) at the Valdrakken portal event. Disbelieving, I skipped re-arranging her bags and immediately switched to my death knight alt and flew her to the one in Durotar. The bag did not drop for her, ending the 4-streak of bag drops for my account.