Isolation transformers produce the rated voltage at the rated current. In order to do so, they must have a voltage step-up function, so at less than the rated load the output voltage will be greater than the input.
Most tube equipment will tolerate +/-5% with little problem, and +/-10% before serious damage occurs rapidly. But of course much vintage gear was designed for 115 or 117 volts input - and most modern gear uses power transformers designed many decades ago, and rated for 115 or 117v input. Those power transformers also put out more voltage than specified unless they are loaded with the specified, usually maximum, load. Given the enthusiasm among many builders for over-rated components, excess voltage is not all that uncommon even if the input voltage is 120v (the current North American standard).
Hard to guess what exactly caused your particular problem - over-voltage is a possibility, but it may or may not be a probability.
