As far as I've read, the reason this happens is because of the Android memory management, which I read was made "worse" because of a better internal task killer in 2.2 Froyo. The app is being marked as not needed in memory anymore and is being killed.
There are a couple things you could do to mitigate the frequency of this happening. There's an app on the market called MinFreeManager which modifies some system config files to change how the memory manager divides up memory to active apps. Get the app and load the preset
Aggressive. Hit both
Apply at boot. Also, you need to be ROOTED to do this.
Another thing I've seen suggested is in the LauncherPro settings, go to
General Settings and check the box for
Keep in memory.
Since doing both of these things, I have seen a reduction in homescreen redraws.