One Motion Sensor Light Turns On Other Lights

I had a situation the other day where three motion sensor lights were all wired to the same circuit, and when one light was triggered it turned on one of the other lights not near it. Can someone explain how that might happen? Does one light coming on create a current surge that might trigger something in the other light?

Those motion sensor/floodlight combos are sometimes separate motion sensor switches and floodlight bases. Its easy to tie additional fixtures into the switched output of the motion sensor. As long as the total connected load does not exceed the rating of the switch, its OK. I've disconnected and thrown away the tacky looking floodlight bases from some motion sensors and have them switching some nice looking recessed fixtures in front of my garage and front porch.

The motion detector switches I bought also have X10 master/slave capability. So, in addition to controlling their own fixtures, they can be switched from a remote X10 transmitter. Or they can send 'on' and 'off' commands to other X10 devices. I have a lamp in my office that comes on when anyone approaches the front of my house.

I seem to remember years ago, reading the instructions, saying not to put more than one motion light per circuit.

Bizarre. To a household 120V branch circuit, each motion sensor/lamp combo just looks like a relay-operated lamp. The solid state circuitry shouldn't be more sensitive to interference from another such device than it would be to any other switched incandescent lamp.