Why is the bottom casing coming off?

Apple will replace this free or send you the part to do it yourself:

MacBook Bottom Case Replacement Program

Apple has determined that under certain circumstances the rubber surface on some MacBooks may separate from the bottom case of the system. MacBooks shipped between October 2009 and April 2011 may experience this issue.

Apple will replace the bottom case of any affected MacBook, free of charge, that exhibits the issue.

Bottom Case Replacement Process

You may choose from three options to get a replacement bottom case for your MacBook. In all cases, your MacBook serial number will be checked to verify that it is eligible for this program.

Assisted service

Visit an Apple Retail Store - Set up an appointment with a Genius.

http://www.apple.com/retail/storelist/

Visit an Apple Authorized Service Provider - Find one here: http://support.apple.com/kb/HT1434

Self service

Order a replacement bottom case kit online: https://supportform.apple.com/201105

You will receive a kit that includes a new bottom case, screws, a Phillips head screwdriver, and instructions for how to remove the current bottom case and install the new one.

If you feel comfortable with the self-service option, you can place your order via the web.

Note: If your MacBook does not exhibit any signs of this issue, no action is required on your part at this time.

Additional Information

If you believe you have paid for a repair or replacement due to this issue, contact Apple regarding a refund.

This worldwide Apple program does not extend the standard warranty coverage of the affected MacBook.

This program covers affected MacBooks for two years from original purchase date. Apple will continue to evaluate the service data and will provide further extensions to this program as needed.