My bike does this under very heavy braking. I have Shimano center lock rotors. It started with only "clicking" for a few seconds after I released the brake. Over the last year it has gone to a constant click when I ride.

I changed the pad and the rotor and the problem is gone.

I think the rotor is too thin as designed to withstand the heat from hard braking with a heavy rider. I weigh about 175 lbs. and ride extremely aggressively. I also only use the front brake in normal riding conditions. As the rotor wears it becomes more susceptible to heat. Eventually it will warp past the point where it will come back to shape when it cools.

So to answer your question, if you are heavy and ride hard I believe this is the cost of having disc brakes. It is easy and not very expensive to fix, so there's that.

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"...the street finds its own uses for things"

Could have a little water in the system.
A good flush and bleed should sort it.
If you are constantly pushing the brake to the limit, try going up a size rotor and make sure that yo are not under minimum thickness.

Water in the brake actuation fluid is not going to cause those symptoms. The water is incompressible, just like the fluid, and all it does is cause the boiling point of the brake fluid to drop. It will also cause corrosion of some parts. In a bike braking system that isn't going to be much of a concern.

There is no way you can get brake fluid in your bike hot enough to boil, even if it has a little contamination, and if it did boil the brakes go mushy.

The problem is the rotor can not cool itself under heavy braking. My front rotor was a pretty black and blue streaked color when I last replaced it. It was still above the minimum thickness for use, but it was warped in several directions.

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"...the street finds its own uses for things"

before you spend your money to replace parts, it might be worth lubing the pistons as it could just be a problem of them not retracting fast enough (sticky pistons). the fix is easy and costs you nothing if you already own some brake fluid. here is a super simple instruction on how to: https://www.youtube.com/watch?v=ILsy2q-1Lqg
make sure to use the same brake fluid as your system uses!

Thanks! All your views are much appreciated. I will give it a shot and clean what can be cleaned there.

Water will for sure cause brake pump up.
It expands at a greater rate than brake fluid.
That plus the fact that your brakes are now overfilled because of the added water will cause all of the lever issues you suffer.
I have blued a few rotors and even managed to melt an icetech one, but never once felt any issues at the lever.

You don't understand how hydraulic disc brakes on a bike work.

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"...the street finds its own uses for things"

You might try taking the spring clip that pushes the pads away from the rotor and “widening/stretching” that a bit.

I understand exactly how they work and deal with things constantly.
You on the other hand are mistaking them for car brakes.