Asked to be a reveiwer after submitting to a conference I recently submitted a paper to a conference. Within days all of my co-authors and I received an email asking us all to select (bid for) papers in the same submission stream from the conference to review. The conference has purposely invited some authors to be reviewers. Our paper has not been reviewed yet.

Is this normal behavior?  Will it look strange to have an accepted paper and be a reviewer for the same conference?
In my field (CS) it is common to have accepted papers and act as reviewer (otherwise all well known authors would not be able to publish at such conferences), but it is uncommon to decide after submission who will be reviewers.

Sometimes it is necessary to recruit additional reviewers because you are not having enough experts for certain sub-disciplines or reviews are cancelled / not in time.