The only 2 ways to "cook" a turbo are 1.) to shut off the engine while the turbo is still hot (no cool down) and 2.) to immediately shut off the engine after pressing the accelerator pedal down, like what old school drivers used to do (vrooom vrooom then shut off engine). The 1st is obvious, the cooling oil stops flowing while the turbo is still hot, causing the oil to "cook" in the bearing. This causes "varnishing" of the oil, leading to smaller tolerances which eventually lead to oil starvation. The second, this causes the turbo to spool up, then when the engine is immediately shut off the turbo is still turning at thousands of rpm and there is no longer any oil flow to lubricate it. Result is a "cooked" bearing.