Pidgin has a feature called auto-away which will automatically set your chat status to away if your computer hasn't been active in a certain time.
You can disable this setting following these instructions:
Open your Pidgin preferences from the Tools menubar
Click on the Status/Idle tab
Change or disable the settings to your preference
Adjust the settings to find out what works best for you. Always remember to set your status to available to cancel the auto-away.