Airpods, 1st gen.

Why do my airpods go into standby mode when I use them for an extended period of time? At least that’s what I think is happening. I can be listening to music or in the middle of a lengthy phone call and suddenly I hear the descending sound but can no longer hear what I was previously listening to and the only way to get them to reconnect is to place them back in the charging case It’s extremely frustrating, is there a way to make it stop?