They are most likely not liable: there is no general legal duty to secure one's own property and/or business. Therefore, to establish liability, you would have to show that it was reasonably foreseeable (or logically predictable) that IF they left their back door open, 1) they would be burglarized; 2) the burglars would then cut through the drywall between them and the adjacent business (yours) to steal from the adjacent business. 1) is probably something you can show or establish: leaving a door open can lead to *that* space being burglarized. But it is VERY rare for burglars to cut through drywall to steal from an adjoining location. Therefore, you most likely cannot show that 2) is reasonably foreseeable, and if not reasonably foreseeable, they would not be liable, since people and businesses are not liable for unforeseeable consequences.