In an earlier version of the game, all the AI were launching colony ships without population and trying to colonize planets with them. They couldn't because the game correctly didn't allow that. The bug was fixed, and I haven't seen anything like that happen since then, until now.
I'm not sure what is causing this because of all the wars that are happening. I have captured colony ships in space that didn't have population, so they couldn't be used to settle a planet.
The reason I'm not sure about the cause is that I have also captured empty colony ships by destroying the shipyard where they were stationed. I'm guessing that the AI built them but didn't launch them. I can't tell if they didn't launch them because they couldn't launch them because they couldn't put population on them, which would be following the rules, or if they chose not to launch them for some reason.
Some of the colony ships I captured in space had a call sign that indicated they were built by a race different from the race that had them when I captured them. Maybe the race that had them had captured them by destroying a shipyard where they were. The AI is now very good at destroying shipyards. Some had a call sign that matched the race that had them, but maybe that race had captured their ships back from the race that captured them when destroying a shipyard.
When the AI was always launching colony ships without population, there were many, many colony ships just sitting near planets they couldn't colonize because they didn't have population. I'm not seeing many, many ships now, but I have seen one sitting near a planet that it couldn't colonize because it didn't have population.

Notice that it is a Krynn ship, but the call sign of M.C.S. indicates it was originally a Manti Cluster ship. They are at war, so maybe the Krynn captured it by destroying a shipyard.
Here is a colony ship I built and launched with population for comparison.

I ask that whoever fixed the bug earlier check to see if it is still fixed. The more I think about it, the more I think that bug is still fixed and what I'm seeing is due to colony ships being captured when shipyards are destroyed.
If that is the case, then the AI is wasting time and effort building colony ships that can't be used because there isn't enough population. Also, the AI obviously doesn't know that a colony ship without population can't be used to colonize a planet.