That's the last backstory shoe that hasn't dropped yet. "For reasons not yet explained, Walter suddenly left Gretchen during a vacation with her family, leaving her and his research behind". We know that Walt's research was crucial to their success. This is the Final Secret, the big event that essentially birthed Heisenberg and Vince clearly been hiding his hand on this since Season 1. I'd be shocked if we find out before the last 2 episodes. IMHO I think it'll be some shit where he did something evil...killed somebody or something maybe....and the buyout was either a deal he made w/ Elliot for his silence, or maybe even a self-imposed punishment because he got a glimpse beneath the mask and didn't like what he saw...basically, a revelation he never really broke bad because he was always *was* bad. But we'll see.