I had the same thing happened to me - don't know when it occurred as I don't enter the TM that much. The only fix I've found is to get the "MacPilot" application (there is a free evaluation period), go to the "General" menu tab, scroll down to and select "Time Machine," and then check "Animate space background." It fixed it on my machine.
I asked about this issue on another Mac forum and didn't get a response. I googled it, but still couldn't find a cause or a solution short of getting MacPilot.
PS - the problem is at least 2 years old, thus pre-dating snow leopard.