I keep auto-save on for every turn, so I preserved the one and re-loaded the one prior to see if it repeated.
It did not. Weird!
Thinking back, I may have had that happen once before, but that was years ago. Wonder what triggered it?
Late in the colony rush, like when the scramble is on for the last 1PQs, I beef up my colony ships with one or more troop modules. This helps against quick influence flipping due to low planet population when my ship beats the AIs to the planet and colonizes. It also makes the ship useful later when the invasions start.
Could the AIs do the same design thing and I simply got to the planet first?