They had Climax and the slopes below it roped off, but the snow didn't respect the ropes. I saw the results of a similar out-of-control avy-control effort back in the 80's, when they bombed the Scotty's-Paranoid face during a big wind&snow storm. The lip came off not as powder but in house-sized blocks that gathered together running down St Anton, failed to make the right turn at the bottom of the gully, climbed the slope and knocked over the top tower of the old slow-double Chair 11. The lift was out of commission the rest of the season. We were staying that week in the Mammoth Chalets above and behind MM Inn (still my favorite place ever for a ski week).
Mammoth's first experience with avalanche in open hours in 1968 is covered in Tracks of Passion, a coffee-table book about Dave McCoy and Mammoth, page 168 --
http://www.tracksofpassion.com/content/11skipatrol.pdf