Maybe someone will jump in and correct me but... could be three things..

1. Your neck is warm and feels sooo good to your snake
2. Snake feels insecure like s/he might fall and is holding on for safety
3. Hugs are fun!

My snake has squeezed my neck a few times, and I truly believe he doesn't realize what he's doing. Hes never been aggressive while squeezing, just acting like a happy snake giving me a hug. I just gently unwind him, and he's fine.

I wouldn't worry about it