Delta has 7 A319s that they use for their charter operations. Since these are primarily used for the NBA, they do not need all of them during the off-season (summer), so they reconfigure some of them back to a standard domestic configuration. When it comes time for them to go back into charter configuration, they will have 108 Y-class seats from each of these aircraft that could be distributed to the remaining A319s in order to add the additional row of Y. This means that next summer the aircraft would either remain in charter configuration, or they would source additional Y seats at that time.

This is all just a guess, but the timing of when the reconfigured A319s start to appear is consistent with when the charter A319s would be going back.