That's what I figured also. There's one more track day left in these tires, then I'll be going to a less sticky rubber. Just for practicalledy purposes.I believe it's caused by the sticky tires. Developers of the Ford GT350 had a similar issue and came up with a design for the exhaust tips with holes to drop the rocks.
I just use my vacuum cleaner to suck them up .If you take a hose and spray under the exhaust tips it will push the rocks up by the back wheels and out on the ground. I live on a dirt road and have to this to the Q about once a week. You can also reach in and pull them out, but I usually just hose them out.