How do I stop this unwanted and super annoying behavior?
The simple answer is to not open the files in QT X v10.3 under Mavericks. Basically, Mavericks has two embedded structures. One (AVFoundation) supports QT X based apps while the other (the 32-bit QT structure) supports classic QT (QT 7) apps. Basically, when you open a file in QT X v10.3, it checks the codec and if it is not one that is compatible with the AVFountation, then the file is sent to the Movie Moderizer structure to determine whether or not the content is conversion compatible. If it is, then the content is forwarded to the Convert structure for the actual conversion. Most content is converted to H.264/AAC but when a codec like Apple Animation is recognized, it switches to the Apple ProRes 422 encoder. Additional codecs like MJPEG video in an AVI container may be sent through the moderizer but the data is simply passed through to an MOV file container without any conversion. The ProRes 422 encoder is now a codec included in the basic Mavericks system codec configuration and appears to be the default "Intermediate" codec replacing AIC as the "poor man's" intermediate codec. AIC, PNG, DV stream, and a number of other codecs commonly used for editing codecs are not supported by the AVFoundation. However, all such formats still function normally in the QT 7/QT 7 Pro Player apps under Mavericks which is supported by the 32-bit QT structure still embedded in Mavericks. I simply use QT 7 as my defualt player for MOV files so they will automatically open in QT 7 rather than QT X when I double-click the file icon. Basically, if a file icon in the Finder is not QuickLook compatible, it will trigger the conversion routine if loaded to the QT X v10.3 Player, so you know what to expect in advance.