The frat bros behind The Chainsmokers’ ASU gig — yeah, the one cops rolled up on this past weekend — are telling TMZ it was chaos from the jump … ’cause even getting the bash up and running was a manic last-minute scramble.

The president of ASU’s Phi Gamma Delta — AKA FIJI — chapter and fellow board members tell TMZ the first house they locked in for the gig straight-up failed, thanks to a plumbing disaster that flooded the yard days before … so they had to haul all the gear out and pivot fast.

They tell us it was so last-minute, the event wasn’t even officially confirmed until 16 hours out — and to top it off, cops were already on the scene before the party had even started.

Despite all the chaos, Jonas McAllister — who set up the celeb campus cameo through his work at a booking company — tells us it all came together in the end, and they would definitely do it again.