He's hitting .294 BA, .375 OBP, 963 OPS, and a 168 wRC+ against LHP so far.
I don't have the team splits, but I'd guess given how trash our offense has been that's probably top 3 against LHP. Happ was awful against LHP when they tried him at leadoff, so they're putting him there because of top 3rd productivity and because it doesn't require them to move any of the everyday lineup players around where they start crapping the bed because they're hitting leadoff, like Happ.
I understand it, but I just don't like it.