A common reason is locking out a brake without using the de-activation tool. The line is capped and the gear has to stay down to allow the wheel to stop spinning. No big deal, just part of the MEL. On a heavy weight take-off, the penalty is quite high, up to 40 tons. Had this quite a few times. The 400 has Carbon brakes and it's rare to have hot brakes, unlike the Classic with the old steel brakes.
Another possibility is 'Gear Lever will Not Move Up'. Again, no big deal, but all done from the QRH, only when the aircraft is established in the climb. A few other possibilities, but the systems are pretty well fool proof. A well engineered and simple aeroplane.