10 Answers

people who like to humiliate others in front of other people suffer from insecurity and are trying to bolster there standing in the group.by humiliating others they deflect scrutiny away from themselves.the way to combat it is to ignore it and continue with what you are doing,most people recognize it for what it is so dont worry.

Only small minded egotistical nits do that

Humiliating someone is public is verbal/emotional abuse.  If someone is doing this to you, you need to stay away from him/her.

Either avoid those kinds of people or break their nose, either way it will stop.

The person doing that definitely has a problem, probably stemming from their own insecurity. There are also those people who see kindness or understanding as a weakness. You are a much better person than me, as I would probably snap, and enjoy making them eat their words. I wish you the best with that situation.

to get attention.

I have known control freaks like the one you describe. They died and no one attended their funeral. No one bothered to see how they died but me and a cop told me that he died from ramming his head into the walls of his apartment until his neck broke. Seemed reasonable to me, I thought he was depressed..

Its called moral support. They do not have the courage to say what they say when it is one to one, and they find the courage to say it in the presence of a third person. The person who says such is a coward in other words!. Like the man who gets his friends to convey a message to his girlfriend that he is no longer interested in her. Also they cannot take a beating in private if it comes to that!. You may find that the recipient is also a person who does not have the courage to fight back when it is one to one.

Mentally ill. Stay away from such persons.

My first thought was someone being helped with something may feel inferior at the moment. For a third party to witness this  causes embarrassment to the inept one.  By diverting attention from his own inability to perform and suggesting, loudly and without objection, that the HELPER is the one who doesn't know what HE'S doing makes it look like the HELPER was actually the one who needed help in the first place. 
If that happens too often, something SHOULD be said, non-confrontationally, but something that will end the BS once and for all.  Something like, "You surely don't think (3rd party's name) believes what you are saying, do you?  Everyone knows you still need help tying your shoes!"  Then laugh, stop helping, and take off!