The drive periodically pauses, checks the burn quality, & recalibrates during writing. Every dip in this pictureâ€¦
(from http://club.myce.com/f86/post-your-nec-optiarc-cd-r-burns-scans-here-279304/index3.html#post2558255 )
â€¦is the drive checking the quality & recalibrating. This means that the drive sometimes changes the way it's burning multiple times, and when this happens, it makes "rings". In that picture, there are 9 dips, meaning you could see UP TO 10 rings.
In YOUR picture, because you didn't burn the full CD-R, you see the first 7 or 8 rings (I cannot tell for sure).
If your burns are fine, it is nothing to worry about. Sometimes users report that their drives begin to do this more often as they age.