For every weight there is a minimum flap manouvering speed. This minimum speed provides a margin of safety as it is creates a buffer zone between stall speed.
If you retract the flaps before you reach the minimum flap manouverings speed you entering into that buffer, or safty zone.
280 was the 747's minimum manouvering speed for flaps zero at that weight.
The pilot probably had a spped restriction below his minimum flap manouvering speed clean and was asking for 280 so he could get the flaps up.