The main rumor focuses on the "altercation with an employee" clause in WWE's official statement on the matter.
Ring of Honor founder and RF Video President Rob Feinstein (no stranger to behind the scenes controversy) posted on his Facebook page that Del Rio was "100% let go" because of an incident that took place at Tuesday's television tapings in Laredo, Texas. His initial post from approximately 7 PM Eastern time said that it was run-in with "an employee who does stuff for the company with social media" and that there "were words exchanged and a slap".
About an hour ago (10:30 PM Eastern), Feinstein came back with more details:
...this was involving a .com guy who works for their website and there are a few versions out there that one the guy said something racist to Del Rio or that Del Rio left catering and did not put his trash or plate away and a comment was made to him which made him snap.
While mentioning that his statements are "just rumors" he says "usually they are pretty dead on".
He then goes on to speculate that Del Rio has been wanting to leave the company since last year, is already wealthy via his career in Mexico and his family's success there - so that whatever the incident was, it might have been a situation where, in the past, ADR would have held his temper or WWE would have tried to smooth it over, but with Alberto basically playing out the string until his contract expires, the decision was made to cut him loose now.
Those theories - that Del Rio forced the issue to speed up his release and/or that WWE used this as an excuse to expedite his departure and save a few bucks - are making the rounds at several different wrestlings sites, all without substantiation.
The other, somewhat intriguing theory is from the "It's a Work!" camp that pops up around any major event, from the Montreal Screwjob to CM Punk's walkout.
Here, the evidence is based on WWE's handling of the matter thus far. One, WWE.com has had the Del Rio release as it's front page banner story all night. Usually, as was the case with Emma's short-lived departure after her shoplifting arrest, releases are smaller items to which the company tries not to draw attention.
Next, his kayfabe name is used in the announcement. Sometimes, as was the case with the original Sin Cara, a performer's legal name will be used. This one strikes me as a reach, as Sin Cara is a property WWE still owns and promotes, so not saying they had released the character preserved the gimmick. It's not like Curt Hawkins was called Brian Myers in the 'black Thursday' announcements.
Finally, in only slightly less of a reach, folks are pointing out that the usual "future endeavors" wording is not in WWE's language. That's could also be because this is being framed as a disciplinary move - perhaps to justify not paying out the remainder of Del Rio's contract.
But that, like everything else here, is speculation.
WWE using this release to advance an angle would require ADR to have anything resembling a storyline, or even a character, in place at this point. It seems like a lot of effort, and a pretty big risk, to put this out there if it's anything other than true.