Assigning presenters the role of a session chair without prior notice

It's pretty common to assign a chair arbitrarily from among the speakers of a session. After all, those are the people who are most likely to want to attend that session anyway. If they assigned someone else, it might prevent that person from attending a different session at the same time, that would interest them more.

It would have been more courteous if they'd asked you privately if you were willing, before putting it in the program. But being chair isn't usually seen as being a big deal, so they may have thought it wasn't worth the bother. Or they may be planning to ask you before the actual conference, but they put some names in the program tentatively, and it was published before they had a chance to contact you.

Of course, if there is some reason why it's not convenient for you to be chair, you can always write to them and decline.

I can only speak for my field (biology), and I can say that I have seen this happen. Most of the time, I think the session chair were informed of their role before.

It is a small task, but you could enquire if they have specific rules to follow or if someone else will be there as a support (technical support for example).