If a German tells you that he was mobbed, try not to look too horrified. He probably wasn't pummelled into the ground by a horde of rabid yobs. What he's most likely trying to convey to you is that he has suffered an incidence of bullying. Not a pleasant experience, for sure, but it creates a far less violent image in one’s head than "mobbing".
Mobbing is another case where an English term has been dragged, kicking and screaming, into German. And it didn't only lose the fight, but also its original meaning. In German, Mobben/Mobbing is used for all manner of bullying, be it at work, at school or in one’s personal life, regardless of whether the perpetrators are a pack of club swinging thugs ganging up on someone in an alleyway or whether it’s one single, mean-spirited person orchestrating a slur campaign behind the victim's back.
I've not yet been able to figure out
a) why another word was required for something for which plenty of German words already exist, for example (and depending on context) schikanieren, drangsalieren, hänseln, nötigen, triezen, zwiebeln, (jemanden) fertigmachen, ausgrenzen, verekeln, the list goes on...
b) why, if the existing possibilities did not suffice, it had to be an English word
c) why they picked the WRONG English word
The noun(das) Mobbing, or, alternatively (das) Mobben, as well as the verb mobben, were added to the Duden (the most authoritative German dictionary) in 1996.
Curiously, the Duden also features the compound noun Mobbingberatung, meaning “a professional counselling service for those affected by Mobbing”. I'm guessing the reason for this is that Mobbing's primary connotation is with psychological harassment in the workplace, with the aim of making someone resign from their job.
I don't endorse bullying in any shape or form, but I wouldn't be surprised if this phenomenon was much more common in Germany compared to other countries like the US and the UK, since it is nigh on impossible in Germany to fire an employee on a permanent contract, even if they deliver a performance that would make an arthritic sloth blush. See here for an amusing blog post which puts a bit of a dent in the myth of "German Efficiency".