Dangerous simmering - exploding liquid? (cheesemaking)
Reducing whey to that level of concentration is going to create what is very likely to be a viscous, non-newtonian fluid.  That means its flow rate changes with sheer stress, much like ketchup does.

The heat at the bottom of the pan will slowly heat up water until a bubble of steam starts to form, and press against the remaining fluid.  At first, the pressure will not be enough to overcome the viscosity of the.. substance.  Eventually, the pressure will be sufficent that bubble ruptures, creating sheer in the fluid, which suddently becomes much mroe liquid.  And so you have a giant gloop.

See related:  Why does tomato sauce spatter more than other sauces?