You have another starting to go bad.
I have had this,after a couple of weeks of normal hassle free starts the plug will have died and you will need to replace it.
Not sure if you use a tester on them if you will find any noticeable difference in readings from the one going bad.
I always thought they just die but I have had a couple do this to me and the light may not even stay on sometimes, but eventually you notice the grey smoke and the engine stumble at start and plugs then fully dead