While acknowledging that with the net the rules have changed, it would seem to me that the unofficial rules of endorsement by a candidate that drops out are still clear: You highlight why you are supporting a remaining candidate AND you avoid throwing barbs at those candidates that didn't get the endorsement.
Hunter has taken an endorsement with limited value (due to his limited following) and managed to disillusion many supporters that stuck it out with him by going with a candidate that seems to be at odds with his signature issue, illegal immigration, and makes a dishonest accusation about another candidate regarding business dealings made by a company said candidate left nine years ago and had absolutely no role regarding the disputed issue.
People have every right to see Hunter's endorsement as sour grapes with a complete side of compromise.