I've never heard of that happening. It's possible the water pump failed and pressure built up suddenly, although I don't know why it wouldn't have vented out into the overflow tank. It's very possible the overflow hose is blocked, or the cap is faulty.
The water pump is gear driven and it's nearly unheard of to have one fail. It's located on the left of the engine crankcase and is dome shaped.
Here is how I would address the problem.
Drain the cooling system and remove the water pump cover. The shaft should be tight with no bearing play. Being it is gear driven there should be almost no play forward or backward if you try to spin it.
Remove your cooling system fill cap and have it pressure tested or just replace it. Blow into the overflow hose to verify it's clear all the way to the overflow tank and air flows freely through the hose.
With the coolant out of the bike, you can now pressure test the system with air to check for leaks.
Take your old fill cap and cut off the pressure spring. Make a rubber gasket to cover the inside of the cap and put it back on the fitting. Remove the vent hose and connect a hose with a low pressure air gauge and some sort of shut-off. You will need a connector so you can attach a bicycle pump. Pressurize the system to 10-15 PSI or the pressure amount shown on the cap and shut off the hose. Watch the gauge for any signs of a leak.
If there is a leak, try removing the oil fill cap on the crankcase and the spark plugs. You may be able to hear where it is leaking into the engine. If there is a leak at the water pump, you would already have coolant in the oil. This causes the oil to foam up and go milky so it's easy to spot.
It sounds like you have a head gasket leak, so I suspect that's what you will find with the test. It's very important to test everything before you tear the bike down. You want to know exactly what caused this before spending money on a repair. You do not want to do this over.
Do you have any questions on how to check those things?
Do you have the service manual?