Think this has to do with saving and keyframes, since saving snaps keyframes back to their start position and unhides them if they are hidden. Most of the time after saving a map that has keyframes, RED will ask you to rebuild before going to texture mode. Auto saving is probably what is causing it to happen if you walk away for a while or just happen more often then needed. I have found sometimes it's just quicker to go file>>new, switch to texture mode there, then re open the map you were just working on and now your in texture mode. Also helpful when you want to adjust something you need calculated lighting to see properly, but RED wants you to rebuild first. Saves the re building and re calculating time. :P