Artillery Bombardment ends turn early!

[Bigwig]

There's a chance every combat round of the turn being burned. This is based on force proficiency - with a good army (70%+ Prof.) you can expect many rounds of combat, with a weak army (I've seen some as low as 30%) don't plan on more than a round or two of combat.

There are other factors - I'm lead to understand units set to "ignore losses" will suck more rounds of combat before they stop firing, and of course you should double-check the turn counter to make sure you don't have an arty. unit which has moved before firing sucking up a bunch of combat rounds.

But all in all, sometimes turn burn just happens, and there isn't always a way you could have prevented it.