If they are not coming from a single source, like a patch of similar objects intersecting our orbit, then they are just a random phenomenon
(x% of all meteors are bright and green). Random phenomena are not perfectly evenly spaced, they appear to show up in spurts, and the ones we don't see (daylight, overcast, out at sea, etc) further chop up the data stream. On top of that, not all sightings are reported, one I saw I told my friends about but never made the papers. I've seen two, by the way, although one was much less spectacular than the other, which suggests they are a part of a continuum of types, not a distinctly isolated and unique category.

The press doesn't report everything, but it tends to seize on minor trends and look for hidden patterns, often where none exist. Something is much more likely to be reported if something similar was reported earlier. A spectacular bolide will generate reports, making additional reports more likely to be get press. Pretty soon we have a major flap going on that is just selective intepretation of the normal background.

This is the pattern of most anomalous phenomena, someone sees something strange and before you know it the Twilight Zone theme is being cued. This is the problem with anecdotal evidence and anomalous phenomena: it's not that there's nothing to it, it's just that the evidence is useless.

Actually, I know the real truth. Most green bolides fall in the Bermuda Triangle. They are messages to Atlantis from the Face on Mars.