cd drive ejects on boot

The historical reason is that any drive that contains
• an ejectable medium,
• that is being examined at startup as a Boot candidate,
• that is not appropriate for booting this Mac --will be ejected.

That implies that the CD drive is being examined as a candidate for a Boot Disk, which further implies you have not set a Startup Drive, or your Mac has lost its setting.

Re-set your Startup Drive with System Preferences > Startup Drive.
Try the same situation right away and it should not be ejected.

Try again tomorrow and in a few days. If the Startup Drive setting is not "sticking", or you lose the setting after a power outage, you may need a new PRAM Backup battery.

plas1, welcome to Apple Discussions.

The problem could be due to some add-on device plugged into the computer. In particular, a malfunctioning mouse or keyboard could be sending a signal to the system to eject the disk.

To solve this problem, unplug all external devices and restart the computer to see if the problem still occurs. If the drive does not pop open or eject the disk, then one of the devices plugged into the computer was causing the problem. Take the keyboard by itself and plug it into the computer, then restart the computer and see of the drive opens or the system ejects the disk inside; if it doesn't, plug the mouse in and restart again. Continue to add the equipment back onto the computer one piece at a time and restarting until the drive opens or the system ejects the disk; the last piece of equipment added to the system is responsible for causing the drive to open.

 Cheers, Tom 😉

My sawtooth does this same thing, but I'm using a brand new logitech wireless keyboard/mouse. As soon as I replace it with an Apple keyboard it quits ejecting on start up. Methinks it's Apple wanting its own hardware!