Most likely this is caused by:
1. Steering being toed "IN", on some chassis with bad tie rod geometry when the front sags under de-cel causing this issue. Even id it is "dead on sitting still it may change with even a little susp. travel.
2. The amount of caster is not enough in my opinion.
Knowing brad personally id say to look at your alignment and tie rods, Brad has build a ton of chassis and this is not normally a problem on his stuff.
If it were me, Id toe it out 1/8-3/16 and try it again. If it helps/fixes it , run it. If it doesn't look further into suspension geo.
Chris