There are many reasons why a glider would bark.
They sometimes bark to communicate with other gliders (or other animals barking far away, remember, they perceive sounds a lot better than us, so they may hear things we do not and reply to them).
They can also bark as a warning, when one feels there is a danger, he will bark to alert the colony.
And sometimes, they seem to bark for no reason (or at least, we will never know the reason)
mines are more effective than a barometer to forecast thunderstorms, if they bark like crazy, I can be sure I'd better close the windows.