So I took my cooper to the dealer (Sewell Mini, in Plano) to get some needed suspension work done, and since it needed an oil change, i just had them do it.

They set the oil change interval to 8k, nearly half of what it's supposed to be.
Any reason for this?

Pretty happy with the service i got from Sewell, but i'm curious about this.

 

Only they know for sure, so you should ask them, and see if their answer makes sense.

One possibility is that your car got an intermediate oil change (between regularly scheduled services) so they re-set the service reminder to their estimate of when the next regularly scheduled service would be.

With such variable mileage scheduled services, it's too easy to lose track of when scheduled full services are due if the service reminder is reset after every intermediate oil change. Leaving the service reminder as is after an intermediate oil change helps to keep track of when the next full service is due.

 

+1 on asking the dealer.

My guess, however. Would be 8k is what that dealer "recommends." I've heard of dealerships recommending intervals other than mfg's recommendation. I also know a lot of people choose to go by the "severe duty" (usually half of normal) recommendation even with top quality oil just as a precaution. I personally have no issues with long intervals.