Withdrawal Liability question

[ERISA25]

I received an odd question the other day: an employer was advised that PBGC has issued a ruling requiring prospective withdrawal liability calculations to be made assuming that all employers are withdrawing at once, like a complete termination. The only fact I am told is that the plan has had 50% of employers withdraw in past 7 years. It is apparently a new PBGC ruling.

I have no idea...any thoughts would be appreciate???