El Capitan: (Text) Cursor moves during my typing
Even though Apple says ignore accidental trackpad input is enabled by default, this does not seem to be the case in Yosemite/El Capitan, as several users have pointed out on the Apple support forums.

To address this issue, I made a program that disables the trackpad for a short time when typing on the keyboard. Take a look here and let me know if it works for you (I've only tested it on El Capitan 10.11.6 so far): https://github.com/thesyntaxinator/TouchGuard