Scoured Valve Body

[Rich3k]

I have a 2004 GMC Yukon Denali with 95K miles. The tranny had a slow shift per the GM dealer. The tranny was flushed with a BG flush at 50K, 75K and was due again at 100K. The GM dealer said the valve body was scored. BG covered the cost of the repair but I am not able to find out what caused the scoring. The service writer is telling me there was debris in the valve body that caused the wear. My question is why did this fail with the preventative maintenance being followed. I do occasionally tow a trailer weighing between 2k and 5K lbs. I use the tow haul mode in drive per the owners manual.