Assigning presenters the role of a session chair without prior notice I am going to present my research on a conference in the near future. This conference is in the field of engineering and CS. It has several sessions in parallel and a total of more than 100 papers.

When I had a look at the program, I noticed that I (and another person) have been assigned the role of the session chair of the session where I am also presenting.
However, nobody has informed me of this, yet, neither have I been asked if a want to take this over.
I had a look at the other sessions, it's similar there: Two presenters in each session have been selected to be session chairs.

So far, I have never experienced something like that.
All in all I am not complaining at all (to be honest, I am looking forward to taking over this task!), however, I am still a bit surprised.
Is it common in other fields or other conferences that presenters are managing their own sessions and are not asked whether they are willing to do so?



UPDATE: I was at the conference and some other chairs and attendees told me that this is indeed common there: They just assign the session chair role to people and sometimes miss to tell them beforehand. Nevertheless, everything went well and as expected this was an interesting experience.
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.