Awwwww...I think its a reaction to those he isn't sure of ..did he break the skin ? He may have been just trying to say - "Don't TOUCH the booty" and lots of dogs aren't crazy about anyone back there - ....Does he ever do this to you or is it just something that happens with groomers and vets ? It's possible a groomer was rough with him in the past and he remembers it.
A long time ago a groomer was rough with Chanel
...she whimpered everytime I touched her side and man was I upset - that's the reason I decided to do my own grooming - it took her a long time to be excited about rides in the car (everytime I took her someplace she'd whimper and it broke my heart)
so to help her get over it - I took her on lots of little short rides so she could get that memory out of her head.