I had a high speed tank slapper with the original 021 Crapstones when they had about 4000 miles wear on them- I blamed the bike until someone in the forum suggested Michelin PR's. I put a set of PR2's (55 rear) and restored my faith in this great bike.
New tires have a tendency to help in matters like that. They aren't cupped and are most likely in better balance. I had no issues like that with my OEM tires, and they lasted 8200 miles on the front and 10000 miles on the rear. Yes, I rode it fast. No they didn't handle as well as my PR4s, but they didn't give me high speed tank slappers.
Quite often the problem lies with bad suspension settings (lowering the bike incorrectly), poor tire balance, incorrect tire pressure, and other issues already stated in this thread. Too low tire pressure can cause damage to the tire itself, damage that cannot be seen, which can cause issues with handling. When you damage cords within the caracas the tire can do all sorts of squirrelly things, wandering and at speed I am sure it could induce a tank slapper. The same thing holds true with bad patching of a tire using plugs. Often the person plugging the tire damages cords while preparing the hole for the plug causing issues.
The tires themselves, if maintained well, should not cause an issue on the C14. Tires do go out of balance with wear, especially the front which tends to cup on a heavy bike like the C14. Getting them balanced as they wear can help.
