there have been discussions about CD brands, and their manufacturing process. some brands (or poor batches) tend to not be perfectly round, and have flaws that aren't visible to the naked eye (slight warping, poor alignment of data paths, etc) which can cause CD's to be unreadable and possibly damaging to the hardware. this would be my first assumption as jostling the computer around slightly was able to get the CD unstuck.
for many switchers though, i find it simply a case of not knowing they need to QUIT any application running that is using the CD drive. Typically they just close the window, thinking it's like Windows and automatically closes the application. This is not the case, and the CD will remain in use until the application is actually quit. Same thing for folders/files in the trash, etc. Restarting your computer will close all open programs, and is typically the best bet when you can't figure out what the hell is going on. Restart, and resume.
buy a reputable manufacturers cds/dvds as well (Verbatim has never failed me.)