Translations:FAQ/125/en

short answer
Increase your class::sendq

long answer
This happens when a client issues a request that will return A LOT of data, usually '/who' (eg /who #channel). If you for example have large channels like 300+, 500+ etc... then /who #chan will return A LOT of data, therefore the max sendq (sendq == send queue) is reached and the client is disconnected... Some clients or scripts do a '/who #chan' on join. This explains why certain people will be killed directly on join, while others will not.