As the other reply said, vesting was 5% first year, 15% second year, then 20% per six months.
As for why: a rebalancing of priorities re work/life/travel balance and a better sense of connectedness to my actual work and the success of the company. Not gonna lie though, I look back at what I left on the table and the AMZN share price over the past 2 years and I still wonder if it was the right choice.