What can casue a radiator to burst? 97 bmw 318ti

The plastic radiator explodes expelling coolants on the freeway.
Temperature is past maximum for some time but was on normal
before I set out for a drive. I stop and check under the hood
and found an explosion aftermath of blue coolants. I check the
side of the radiator and it looks like Niagara Falls when a jug of
water is poured into the radiator.

Today, I had the thermostat pot boiled and it won't open. I
don't know how to check the radiator cap but I assume it's
dead. Some electronics were also destroyed from the splashing
coolants. I wish I knew why the radiator exploded. Was it the
heat, the thermostat or the cap?

Thanks