Question
What would cause the Solenoid to get hot?

Answer
Thanks for your question.
A few things cause the solenoid to over heat. The water could be off, in which there would be no cooling water flowing by. A bad solenoid could heat-up, but usually the controller would shutdown that zone or blow it's fuse. A partially bad wire or splice could cause low voltage. If the voltage is low then the amperage goes up. High amperage causes heat.
I would test the terminal voltage at the controller if OK then test the voltage at the solenoid. You should read 24v-28v.
Matt