This is the strange thing: my last dog, Luke, did almost the exact same thing (at about the same age.) There was a huge after-rain mud puddle in the middle of a ball field where the dogs were playing. I started to walk out of the field calling to him over my shoulder thinking, like he always had, that he would just come running. I was late for work, and he was coming with me... Finally, I looked back and saw him look right at me, look at the yellow, sandy, muddy puddle, look back at me...and, well, you can guess which he chose...He dove into that mess with pure abandon.
Yeah, I was pissed then---mostly because we were late, and I didn't have time to clean him off. So into the car he went, with just a quick towel-off...
Maybe it's an adolescent thing?! You saw the glint. The glint of defiance. The glint of...pure abandonment to the pleasures of the flesh, owner be damned!
P.S.: His surgery is scheduled for Wednesday am. Good thoughts!