More the latter. Someone did a breakdown elsewhere, but Horton's current lack of Ks can be attributed to his fastball. Hitters in the majors seem to be doing a good job of picking up on it and making contact (even if it's foul balls), so it's not really a strikeout pitch despite its velocity as things currently stand. As a result, he has to play off his fastball with his change (which is devastating) and his breaking pitches (which are still a bit of a work in progress), and if his fastball isn't working super great, everything else falls apart.
I'd be really, really, really curious to see what the PitchF/X has to say about it, but my hunch is he has a bit of Kyle Farnsworth in him, where his fastball does not generate a whole lot of movement and he has to rely on location with it to avoid getting walloped in the zone.