Are feeding a full-wave rectifier with it to charge batteries?
Do you have an understanding of peak-to-peak AC voltage, RMS AC voltage, and DC voltage?
In other words, the transformer is rated in RMS Volts, which, when rectified, under light load, would "look" more like the peak-to-peak value of the AC that was rectified, less voltage drop across the diodes... likely HIGHER than the nominal AC voltage output rating of the transformer.... which is EXACTLY what you are seeing.
U need a voltage regulator and/or a slightly lower output voltage transformer.
Clear as mud?