I think the reason is thus:
V1 cannot be less than Vmcg. At high alt a/d's (NBO;HRE;JNB etc)due to density alt Vmcg is slightly higher than at sea level (basic gas laws)so V1 has to be increased correspondingly, and as Vr is a function of V1 that goes up too.
V2 is a function of Vmca, which also increases with altitude.Thus you also have higher V2's at high a/d's.
I remember having to add a few knots to the T/O speeds at airfields >3000 amsl on various types.
...not a particularly scientific explanation I know, but I hope it's helpful! DCD