1) Is there an easy way to decrypt my HD? I have personal files locked in this encrypted drive to which my work (non-admin) userid does not have permissions.
Yes - get your administrator to set a policy not to encrypt that drive letter, when the system syncs, it will decrypt the drive.
2) Is there any explanation as to what could have caused this, or better yet, is there a way to prevent this from happening in the future?
Your company set a policy to encrypt all drives on your laptop. They obviously want to protect their data from getting into the wrong hands. The answer though to stopping it happening in the future is the same as above (or, don't plug it into your work machine).
If your admins are uncooperative, you may be able to plug-in USB drive and transfer all data to it. Then remove both drives and reformat encrypted one on your home PC (may need to fixMBR it first). If you are computer savvy, you may be able to take affected disk image, store it on USB drive, then restore back to affected hard disk (restoration should be done on home PC to avoid re-encryption again).
1) copy your file to computer then format your removeable drive or the most easiest way is check with your administrator for your computer policy and untick your drive to be unencrypted. after sync your drive will be decrypted.
2) check with you encryption adminstartor potentially it having policy to encrypt all drive.. normally the policy will not encrypt removeable drive - check with your administrator on this.. sometimes there also an issue when removeable drive detected as normal drive by the system and get encrypted.