Negative voltage regulation in transformers
It usually boils down to leakage inductance. Leakage inductance is in series with each winding and it's that inductance (usually small) that doesn't couple either primary to secondary or secondary to primary. In a normal power transformer, the problem of leakage inductance manifests as: -


Vout/Vin not exactly agreeing with what the turns ratio suggests
The above problem getting worse under load conditions


So, it's a series component and can resonate with an output capacitor and here's a simplified example: -



L is leakage inductance and C is the capacitor you loaded the secondary with. R is the equivalent copper losses. Depending on several factors you may get a very big resonance peak. Examples vary like this: -



If losses are very small, many dBs of amplification can be achieved. The graph above is normalized at 1 Hz just for convenience. Q is the quality factor of the leakage inductance and depends on resistive losses and can also be influenced by core losses.