Due to the current spread of traffic GMCs 1+3 are bandboxed as GMC1 (121.9) is too quiet to warrant putting a bum in that seat. The vast majority of start delays are due to bad planning from the airline who occupies T5. They seem to allocate stands to aircraft which all have the same EOBT +/- 5mins and then expect them all to push when they are ready. The other day in delivery I had the entire Y cul-de-sac call for start within 5mins, obviously this cannot be done and so regulating the startups (delaying) protects the ground controller from overloading.
If this was better planned then I am certain there would be less delays. You may see 121.9 open more often after UA/NZ move to T1 tomorrow...who knows!