I also had the AI use that tidal wave spell on my army that was about to attack a fringe town (though it was lvl 3 town) and it had frozen me the turn prior. Good to see the AI tossing spells around.

That's because the Frog got an altar

I've been finding that the AI casts a lot of spells on me strategically but the game doesn't always do a good job informing me that this has happened. I'd like there to be a notification.

I've been seeing notifications over on the right where you get notices about units trained, improvements completed, etc.  It tells me "Magnar has cast Antipathy on xxx" and "Resoln has cast Wither on yyy".

If you time it right and follow one of your caravans into the enemy territory, the enemy will cast freeze on your caravan! 

In my current game, the Quendar custom AI demanded tribute from me, and I almost turned him down. Luckily I noticed the full stack he had sitting just outside the border of my undefended, nearly developed Conclave, and decided maybe I could spare the gildar.

Coincidence?