Water Line HELP!

I need some advise BIG TIME! Long story short, my brother and me purchased a second floor condo to renovate and flip. During our demo, we left the trap pipes in the bathrooms open. According to our condo association, water came out the trap pipe from the condo above us and caused a leak into the downstairs condo which we are responsible for. This does not sound right. I've never heard of water traveling up and out the trap pipe unless there is an underlying blockage somewhere down the line. Any ideas??? Thanks a ton!!!

I'm with you water doesn't flow uphill must have been a blockage

If the stoppage is below your unit but above the fixtures below
it can back up thru your fixtures there are rubber caps or plastic inserts
that tighten in the drain

Thanks Geofd for the reply. The only way for water to back up through the traps would be some kind of blockage or restriction below, right? I'm trying to argue my liability.