Why would the switch to my living room ceiling fan cause my porch light to come on at night?

There is a switchplate by my front door that has three switches. The left one controls the celing light in my living room. The middle one controls the ceiling fan in my living room (in a separate area). The right one controls the outside porch light, which is a "motion-detector" type of light. (When switched on at night, the light comes on if there is motion around it; otherwise, it stays off.) I keep the porch-light switch "on" at all times so it will go into its night mode at dusk.

Recently, I have found that, sometimes at night, when I turn off the ceiling fan to go to bed, the porch light will come on, as if it has detected some motion. ??? I do not understand why, or how, it is doing this -- and it doesn't do it all the time.

Can someone knowledgeable in these things let me know what might possibly be the problem, and how I would fix it if it is a big problem, or whether I can just pretend it isn't happening and not worry about it???