In the Performance manuals for the 744, holding speeds are increased by about 20 KIAS at 20,000' and above. I suspect it has something to do with stall margin at max angles of bank (since holding info includes all margins for the turns), but there is no explicit explanation.
The Boeing FCTM has a slightly more elaborate explanation, which addresses margin to buffet at full maneuver capability (as I indicated above). I suggest you check your FCTM.
Last edited by Intruder; 26th Oct 2016 at 23:51.
Reason: FCTM reference