Apple - When will Finder use large amount of memory?

This inconvenience is caused by a finder prefs entry:

"show in new window: all my files" this is consuming too much mem. Select a different folder and all is fine

I can't say much about preventing this, but there is no need to reboot. Just kill Finder, using Force Quit from the apple menu or killall Finder in a Terminal window. A new Finder will then be started in the old one's place.