From time to time BitLocker enabled machines enter recovery mode, requiring an unlock to proceed. Sometimes this comes as a surprise to you as an administrator. An example of when BitLocker  unexpectedly enters recovery mode could be when installing a language pack as this modifies the boot configuration data (BCDEDIT) making BitLocker automatically go to recovery mode as it thinks this is an attack on the machine. Ask the Core team has created a list of the most common problems with BitLocker entering recovery mode.