Their key markets (ethylene/polymers) have been on fire for over a year now and they reduced structural costs a tremendous amount. They have generated a ton of cash since emerging from bankruptcy and needed to either unload it or buy something. If they didn't they likely would have been a great takeover target with billions and billions in cash on hand. Also, they were/are owned by a number of investment companies who provided the backing to emerge from bankruptcy. These companies wanted some "payback" on their investment and got it promptly with the "special dividend".
This was not a spin-off company. The history is that it was LYO (Lyondell) prior to Dec 2007 and merged with Basell and went private as all shares were bought to form LyondellBasell. Then ~ 12 months later (Jan 2009) they went into bankruptcy and emerged in May of 2010 as a new company (LYB). They've been kicking but since.