A320 battery switch off requirements
As above!
The 2min wait coincides with the APU flap being closed. (Ps: 2 min is worst case scenario as APU continues to run for 1 min after use of APU bleed)
However it is the actual de-oilling sequence that is the key factor. The flap closes when the de-oilling sequence finishes, hence the association.
Also when APU start is attempted with apu flap still in uncommanded position (ie. incorrect shutdown) this could lead to APU start fault. (Hence a little wait after master on does not hurt)
My 2cts