Since this is Walmart it is probably a multi-partition Bosch Radionics system. If the security system is set off by the fire alarm then this is the result of bad programming. I have heard that fire codes prohibit the use of a security siren as a fire alarm signal as it is not intended for the purpose.
However I have also heard of other cases where the fire alarm trips a fire zone on a security system so that the building owner can save money by not needing two monitoring accounts and extra phone lines for the fire alarm. Unfortunately, though, this opens a whole new can of worms when it comes to fire alarm reporting as normally this means fire troubles do not get reported.