Probably a faulty proximity switch on some portion of the gear retraction logic, maybe the airplane didn't know the I/B gear door was open? @MikeD is right about gear pins, the whole point of their existence is to physically not let the gear move from a down and locked position when they're installed. The gear on the airplane in the video started to retract and stopped, newer airplanes are easier and harder to work on simultaneously.What would this be? A landing gear control unit failure partway through retraction? Some sort of WoffW switch failure at a similar moment? Heavy farts from coach?
It's all ball bearings these days...probably a stuck Fetzer valve