Neutered Dog Humping

Question
I neuter my dog within his first year.  He had not humped before and did not hump after (he is now 5).  But that changed the other day when I had some friends over (he's not used to seeing them).  One of my friends would not leave him alone the entire night.  She was constantly picking him up.  He loves attention but she seemed to be irritating him.  Anyway a few hours later out of nowhere he starts humping her leg.  He had never humped before and he has not done it since.  I don't understand why out of the blue he started humping.  What does this all mean?  Right now I'm afraid to have her over again.

Answer
Neutering reduces humping not because it reduces interest in sex, but because it reduces interest in dominance.  Your friend may have been signaling him she was higher ranking than him.  Picking him up could be part of the problem.  He was signaling her he doesn't accept her dominance.  She needs to avoid looking him in the eye, patting the top of his head, and letting her teeth show in a smile.