Unexplained activation is most frequently caused by spikes on the powerline. I've never seen reports of transceivers (TM751, RR501) being vulnerable. Motors, old style magnetic ballasts for fluorescent lights, etc. are frequent culprits. One type of smartmeter (and only in one region around Cincinnati) causes random activations and I suspect its due to the same design flaw in some X10 switches as it puts very high amplitude signals on the powerline which may function in a similar way to spikes. Noisy CFL power supplies have also been blamed for similar events.
You might check to see whether any neighbors who share your distribution transformer use X10.
Brownouts (i.e. momentary power dips) can also cause X10 switches/modules to reset, resulting in unplanned activation.
Things like this are extremely difficult to trackdown without test equipment so any details you can provide on your exact setup such as switch/module, computer controllers, etc. model numbers may be helpful.