who turned in the alarms? why?

Mildred's friends turned in the alarm first, but Beatty disregarded it. Then Mildred sent an alarm as well.

“Was it my wife turned in the alarm?” Beatty nodded “But her friends turned in an alarm earlier that I let ride. One way or another, you’d have got it.” Said Beatty.

They turned them in because of Montag's unorthodox behaviour and possessions.