interesting observation. Since you mixed yeasts its difficult to tell if the increased esters came from the fresh or old yeast.
I don't think old yeast slurries perform well and in your case, where a new and pure culture is available, I suggest propagating a full pitch from that. Even with old slurries I suggest propagating from a small sample if the purity of the yeast can be trusted.
There are lots of contradicting statements regarding ester creation by yeast. One thing that is common to all is that acetyl CoA, an intermediary compound that the yeast produces when it metabolizes sugar, becomes available for ester production. That can happen when the yest produces more than it can use for its growth. An "overflow" of this compound into ester production can be caused by many factors including yeast health, I think.