What would be the cause of a spun rod bearing?

The motor is a 418 small block that has been together since the beginning of 2008. Probably has well over 1500 passes and 1000 miles on the street. The bearing were inspected and look good around January 2012 and they looked good. I run a 7 quart Milodon pan with full groove main bearing and a high volume oil pump.

Last Saturday on my second pass the car felt slow and was slow to pull to 6500 rpm. I went through the traps and lifted and heard a bottom end noise. I pulled the motor apart Sunday and I had spun the #4 rod bearing and pushed it out towards the crank. When it lost the bearing it hammered the rod and bent the rod cap. All the main bearings looked fine.

Why after all of this time did it spin a bearing?