Word. I heard they displaced with water before doing a negative test on the cement plugs/wellhead. They probably did that because it makes things go a lot faster when it comes time to leave location. When they displaced with water they lost a lot of hydrostatic pressure and well came in. I think I remember reading they were using 16.0 ppg mud so at 5,000 feet that is 16.0 x 5,000 x .052 = 4,160 psi at the BOP stack from the mud above. When they displace with water it becomes ~8.4 x 5,000 x .052 = 2,184 psi. They effectively lost 2,100 psi of pressure that was holding the gas down, so that shit came up.