The on-access scanner feature of Sophos Anti-Virus versions 8.0 - 9.1 may cause OS X to stop responding and unexpectedly restart. If this happens, you are presented with an option to move related components of this application to the Trash. Doing this disables the on-access scanner software to prevent this issue from happening again.
For more information, contact the developer of Sophos Anti-Virus to see if an update is available that is compatible with the version of OS X you are using.