Most likely it's the brake light switch. Subaru had a big recall on that year
of cars for faulty brake light switches, so that's where I'd start looking. It doesn't list the Outback, but it does list a bunch of 2013 Subaru models.
I've done two brake light switches, and they were cheap and easy DIY fixes. If it was me, I'd replace the switch and see if that fixes the problem.