A couple things might be:

Hitting a pot hole with a particularly nasty sharp edge.

Or,

Running at higher speeds with low tire pressure, overheating the tread causing separation.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250