Why is OS X indexing *again*?
I found my disk was indexing a lot a few months ago, probably more than twice a week. Finally tracked it down to Backblaze - it turned out that backblaze creates a lot of temporary files as it backs up, especially if you're working with lots of small files (which I do, being a programmer). Spotlight would see those changes and initiate a re-index causing my fan to kick in and the laptop to get hot and burn through battery-life very quickly. Adding the /Library/Backblaze directory to blacklist in Spotlight's settings fixed the problem.

I've also heard people can have the same problem if they have a very active Dropbox account.

If you're doing any video editing this can also be a cause; adding the working directories to Spotlight's blacklist should help. Or, basically, any app that creates a lot of files outside of the OS's temp directories.