yes, it's not usual for apps on OS X to cause system shutdowns, but it might happen when an app uses a library that loads kernel extensions / drivers. In our case this library is OSXFUSE. Would be cool if you could send us all BoxCryptor and OSXFUSE related crashlogs. Still this shouldn't happen too often. The problem was fixed for you most probably because the updated version of BC also installed a newer version of OSXFUSE.
If you are willing to send us the crash logs, please grab them from Console.app and send them to this email: ag [-at-sign-] secomba.com