Who pays plumber's bill: HOA or owner?

We own a condo in Denver, Colorado. Our tenants informed us that they started a load of laundry and water started to come out of the drain in the water heater closet. They were doing a dark wash load, but the smell from the drain was bleach. Our unit is on the third floor of a four story building. The tenant called our property manager and the HOA to have a look at the issue. The HOA called out a plumber who said he snaked the "main" drain for all 4 units down in the garage. Water had apparently leaked down to the garage level. The HOA called a company to come do some drying of the units. My questions: Who is responsible for these main drains that are connected to all of these units? Who is responsible for the HOA bill (escorting plumbers)? Who is responsible for the the drying machines? Who is responsible for the plumbers bill?   We have our own policy to cover the contents, but our policy doesn't cover sewer back ups.