Whatever the exact reason it is 99% certain that it's due to a mode set on the channel, some of which have been described above.
Nowadays many networks have their own unique modes (+R, +M, +C, +c etc etc) - We can't know which network you're using, and can't memorise which network has which modes (due to the number of networks there are). The best thing to do is to ask in the network's #Help channel and see if they can suggest all the modes it might be.
Also try checking out their website, they might have documentation on non-default modes that network has. If you don't know the network's website, search
Google.
Best of luck!
Regards,