Upper Framing Plate Damage
It's hard to imagine what caused this upper framing plate
damage. My best guess would be the roof flashing around the water heater
vent pipe was damaged somehow allowing moisture into the water heater
Most people would assume that it was the water heater somehow just for the simple fact there is water in the heater and this would make sense to them.
From my experience providing home inspection services as a courtesy to some of my potential clients for remodeling work I have noticed a lot of assuming when it comes to problems like this. Some know it all's well everything to you even if they don't have any idea what they are talking about.
Most of the time water does not go up but often goes down , especially in a situation like this. Be careful when inspecting to make your best educated assumption and not just a random guess.
The more you learn about what causes certain effects to home damage the more accurate these guesses will be as you gain experience in the home inspection field.
Another cause for this upper framing plate damage would be a water leak in the pipe that would actually have sprayed onto the wood framing and over a period of time would have caused wood rot and attracted termites.