In a nutshell: kegs that have been eaten by the wurm will trigger placed kegs to despawn
Kegs placed at cobalt will despawn after 2 minutes if a vet wurm doesn’t make them explode. Once the wurm eats the kegs however this despawn timer set for each keg is not canceled however, so once 2 minutes have elapsed a keg that has been placed will then despawn before its own 2 minute timer is up. This can lead to some rather farcical situations where the zerg starts placing kegs, has nearly enough down only to suddenly be sitting at zero kegs. While strategies have been developed to work around this, it remains a hidden mechanic that greatly hinders the ability of inexperienced players to complete the boss. This behavior is so obscure that I have even run into experienced members of wurm guilds who had no clue how it works.