Whether it's an actual explanation or a handwave, I don't think it really matters to me. The idea of everyone doing their own thing and only teaming up for the biggest bad guys is pretty well-established comics fare, even though it doesn't make sense.
Based off the trailer it appears that Thor is going to face some form of elemental, primal, eldritch force that is unstoppable (Well it kinda has to be to threaten Marvel's version of Superman). This force is so much a threat it threatens Earth and that is so bad that Thor will turn to Loki for help.
If this threat is truly world busting then why isn't the rest of the Avengers involved? (You don't have to answer this, this is rhetorical.)