Is it? You've taken two facts (1. Interface redesign and 2. 50% growth last year) and assigned causation to the first. In my persepctive, reddit has been digging into the cultural psyche for a long time and has been growing steadily. The news uses it to replace local journalism now. Anecdotally, a lot of people I know are now talking about things they saw on reddit (I've been a user for ~10 years). I think the interface design is not relevant to growth. If anything, it probably decreases it, although I think the type of users they are attracting now don't care as much as people who post on HN do.