I think my hub problem was related to a tweak issue. I didn't know how to adjust tweak so when I was setting up the cars, I just screwed down both sides until they looked even
Luckily another member on this site explained it to me in a PM and after checking (and fixing) the tweak, I didn't have a problem with the hubs coming loose. All 3 cars were hooking pretty sharp in one direction before fixing the tweak. I think this combined with a ton of traction was slowly working them loose over time. I ran all day sunday and the hubs didn't loosen up once.