<human>: Hey r/dogtraining I'm dealing with a pretty bad issue with my 2yo 55lb Pit/Lab mix Casey and wondering if any of you has experienced the same problem.

A little background: When I adopted the dog (at about 1yo) I noticed he was playful and rowdy with other dogs, but very cautious and worried around people. He was very shy, but sweet with my girlfriend and I so I decided to adopt him. During the first 6-7mo I had him I tried to socialize him as much as possible by taking him to dog parks, friends BBQs, etc. He was also pretty easy to train commands like sit, down, stay, etc and tends to listen to me well. He was always shy and cautious around strangers, but it was never a problem until recently.

Now whenever we have strangers over to the house it's recently escalated to snapping, growling, barking, and nipping. This seems to happen mostly with movement (stranger walks forward, reaches/leans for something, or moves an object). Most recently, there was an incident where he nipped a guest fairly hard a drew a spot of blood. Needless to say after that, and the new behavior he's exhibiting, I don't allow him around strangers anymore unless he's on a leash and it's scary/embarrassing when we have people over to the house. He's also very unpredictable in that he's totally fine with some people (and generally ok with most strangers outside of his environment). He can go from wagging his tail to snapping within a split second. I think much of this could be related to him not being properly socialized with people during his first year as a puppy, or perhaps he's trying to protect us from strangers he deems as threats?

I'm having a professional trainer come over in a couple weeks to address the situation and in the meantime keeping him separated from strangers for safety concerns. I'm just wondering if any of you have experienced this - I've been reading a lot of stuff online, but everyone seems to have a different technique and this is a pretty complex behavioral issue that's a bit over my head.

Could you distill the essence of the text into 1-3 sentences?
<bot>: Casey is displaying what I think is fear/territorial aggression towards strangers. Problem is escalating to the point where he can't be around new people. Not sure how to proceed with fixing the issue.