My German Shepherd male is 7 months old and has been socialized and exercised since he was 10 weeks old with trips to the local dog park (almost daily).
Recently, his behavior has changed toward one of the dogs he has played with since he was a pup (a female Husky). After brief periods of typical wrestling and chasing (the GSD chasing the Husky), my GSD will grab the Husky by the neck fur/skin and not let go.
The Husky will sometimes yelp, but it does not deter his continuing to hold on tight. Trying to separate them results in the GSD holding on tighter. Its as if the GSD is in some kind of zone, because verbal commands which he otherwise is good about responding to, don't work.
He does not do this with any other dog playmates.