I assume they just selected the gear up at the usual trigger point for selecting it down. This is assumption here, but SOPs, practical drift or perhaps the reality of repetitive line flying may dictate that in normal circumstances, the gear comes down at 1700 ft or whatever, this would have triggered them to reach and move the gear lever, without thinking what they were actually doing. Due to the panic/overload they were paying no attention to what they were doing and that is where the slips and errors just start to cascade. That is the only reason I can imagine this happening. Sure it has been done before but without these awful consequences.