LOT Airlines in Krakow asked me for a second form of ID beyond my passport. Is this normal?

I'm a US citizen and was checking in at the LOT Airlines counter at Krakow Airport. The agent asked me for a second form of ID in addition to my passport. I happened to have my driver's license on me, which I almost never travel with since I usually just carry my passport.

I've been to 40+ countries and this is the first time I've ever been asked for this. My passport has always been sufficient everywhere else.

Why would an airline ask for a second ID when a passport is already sufficent? And my name on my ticket matches my passport.

In addition, If I hadn't had my driver's license with me, what would have actually happened?

Thanks.