Population Reduction

[henry k c]

I'm in an epic game, as the dutch and declare war on portugaul. I have 2 horsemen heading for Coimbra. The first horseman enters a tile 1 square from the city, no problem, but then when the second horseman enters the same tile, Coimbras population automatically drops from 3 to 2, does anyone know what causes this?

 

[TimBentley]

They poprushed a defender during your turn.