The keyword here is that GP never said "if you do fire junior people", but "if you are firing junior people". The later implies a recurrent pattern of behavior and, unless you want to argue that an unknown factor beyond your control has been sending a disproportionate amount of bad apples your way, it is ultimately your problem.

Most common causes I can think for this problem are: an indiscriminate hiring process that fails to keep out poor candidates, either defective operations or hostile work environment that results in excesive burn out rates for employees, irrational expectations from management teams (specially founders who struggle to make the transition to later stage startups).