Why CDRW trays opens

On a stand alone machine,my cdrw tray opens. It will open on boot up and then sometimes while the machine is running. Once it opens, it remains open until I close it. I can close it and later it will open again.
The OS is W-98, I have one CD and one CDRW. The CDRW(Memorex)is the only one that opens on it’s own. I’ve changed the cable from IDE master to piggy-backed on the hard drive. Nothing changed, it still opens. It copies CD’s properly. What is causing this problem?