I agree. The drive pressure is pushing it back shut.
Actually, drive pressure is trying to push it open at all times. The only way for drive pressure to hold it shut would be to have pressure coming back from your exhaust after the turbo.
What I mean by drive pressure is, the pressure in the up pipes is so great, it's spinning the turbine wheel faster/harder even with the waste gate open. The gate doesn't care about drive pressure, only boost pressure. So if you ran the wastegate stock, a big ass tune with lots of timing, the waste gate would be open but because the stock turbine housing is so tight, that little hole from the flap being open isn't enough to bleed off drive pressure and drop boost. Instead you keep building boost to the point it starts to choke up. That's where up pipe bellows start blowing.