No idea. To be honest I don't know what basis they could have DQd him on. I thought the rules were quite clear, you race the series in whatever cat you start it in. The rules explicitly state that riders who move up a cat can continue to compete for the rest of the series!
I think it would be something like this ...... I'm C and race about 3 - 3.1 w/kg. My HR is always up high so if I squeeze into B by pushing just over the limits I'd be OK as I've never shown that I was a B sandbagging. But, if I had been suspected of cruising in C then jump to 3.4/3.5 w/kg in a race then I was clearly always a B and just sandbagging.
We've seen this before in DBR and IRL; in both we had a rider who was winning easily in C but staying just in the cat limits. Then they did a crit that accidentally bumped them up to B. Their next B race they were pushing 3.5+ w/kg (one guy was pushing 3.7) for an hour so they were clearly always a B but holding back in C to win.
It must be really hard for the organisers to spot, but there are loads doing it. On the FB page at the moment there are many C's complaining of riders winning with A numbers, but these guys were 'C' going into the league. Crazy!