Some alarm systems are wireless instead of wired. That means that somebody can listen into radio signals and spoof commands to the security system. Lots of systems from big companies like ADT employ wireless signals, and somebody can disarm these systems remotely with some high tech tricks.
Of course, it is also possible that somebody could disarm your system if it is wired too if they bothered to splice into the wire, but that is harder and then they would have to creep around your property. I submit that it is better to have a wired security system for this reason.