Who is responsible to pay the plumbing invoice?

In A Condominium with 9 units that had 4 units having sewage backing up and 2 owners called plumbers.The problem found and fixed was the main drain connecting all units was plugged and fixed .The condo board sent invoices to only the 4 that had the problem and my question is .Should the invoices be paid out of the condo complex fund? WES