Why would a carrier terminate my cell site? The answer is quite simple; they don't need the site any longer. When there is a telecom merger, buyout or insolvency, there may be a redundancy of service in the area. The service provider may choose to save the extra cost by eliminating one or more of the redundant cell sites.
Check your lease and see this clause or something similar. "This agreement may be terminated, without penalty or further liability with ninety (90) days written notice...".
These cancellations don't happen often, but they do happen. AT&T bought out Cingular in 2005 and bought Bell South in 2006. Verizon bought Alltel in 2009, and the Sprint and T-Mobile merger has recently been approved and will add to these redundancies. Each of these caused and will continue to create terminations as they weed out the extra and redundant sites.