How did this happen, in that all I did was install TextMate from the download link.
When you press Space with a file selected in the Finder, you activate something called QuickLook that's built into MacOS X. It has an open, plug-in architecture that lets applications provide viewers for all manner of file types - they can't possibly all be built into Mac OS itself, as new kinds of files are invented all the time. Instead, applications provide QuickLook plugins. You can also find standalone QuickLook plugins for certain files online; for example, there's one that will let you preview the contents of a Zip file.
TextMate provides QuickLook support with syntax colouring in the preview window for file types it supports. This is included inside the application bundle; no operating system modifications are made. I'm not aware of a TextMate setting to turn this feature off if you don't like it, but in theory you can just delete the QuickLook plugin from within the application bundle to do that - for example, see:
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TTFN, Andrew Hodgkinson
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