They're trimming their teeth. They normally keep them trimmed by simply chewing on hard nuts, acorns, etc... Certain times of year when those are harder to come by they'll chew on other things like siding, posts, fencing, or even bars. They have found a wood that works well for trimming and will continue until something better comes along. It's probably only one squirrel or one family. Trap them and you get rid of the problem. I doubt if all the squirrels in your neighborhood are stopping by. That said it is an outdoor bar, and there could be some squirrel raves going on. If you start missing PBR and cocktail umbrellas you've got a real problem on your hands.