Ebbie,
You'll probably get smarter input than mine, but occasionally my ear/mind 'hears' other parts that ought to be in something we're playing. For me, it's usually guitar fill that I can't do or sweet fiddle stuff.
I tend it by throwing in an occasional 'yip' when one fits (which then turns my head to wondering why Bob Wills did so much yipping/hollering) but I can see how it could easily translate into my hands instead..maybe that's what your guy is doing?

If he's exceptional with his interpretations, maybe he's just flukishly musical and needs to whack it out? If his pounding was notes, it might fit with the rest of the music nicely.

Just a guess though.