It could be a requirement set by AT, rather than P&WC and if the case, I suspect to reduce the likelihood of a hot start, as it allows the pilot to ensure they have good NG speed prior to lighting it up. I was always a fan of holding back on opening up the fuel tap, until the NG was higher than published minimum data, so as to get a much cooler start. Of course, a good battery and preferably external power makes this a lot easier.
Additionally, the AT has a nose high attitude, so unburnt fuel that does not drain out, is likely to run down the GG case and sit around the exit of the lower diffuser tubes, just waiting for the next start.