Chainsmokers ASU Frat Bash Was Last-Minute Scramble, Duo's Footing Cop Fine

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.

As for the fine the cops slapped on them, they confirm to TMZ that The Chainsmokers' Alex Pall and Drew Taggart are picking up the tab for the citation -- and the fraternity is seriously grateful, since it could’ve set them back anywhere from $1,000 to $2,500.

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.