Nothing definitive from the experts. My bet is on love.
The two times that I've seen this behavior, the aggressor appears more reddish in coloration.
Yes, I said
two times.
Oddly enough, since the post to
inaturalist.org, I witnessed another occurrence of head grabbing by a pair of lizards.
We were out foraging for spring kings and morels, which are popular with the wild mushroom crowd, when I spotted another pair of alligator lizards doing the same head grab behavior. Although the same size, the dominant lizard is above to drag the lesser along with speed and without resistance. It's almost like they have a "Spock" grab on the submissive lizard. Unfortunately, this fast moving couple escaped under the bark of a fallen tree before I could retrieve my camera.
It was really interesting, because we were at 5,000' elevation, almost twice as the 2600' elevation of the other pair of lizards I had photographed and sent on to the reptile curator at the L.A. Museum of Natural History.