http://my.opera.com/haavard/blog/2013/02/13/webkit
Opera has made an official announcement that they will promote monoculture through switching from Presto to WebKit.
This is because their users consist of only 1% to 2% of the market while most of the market users are using WebKit based browsers such as Google Chrome.
Thus, Google had sort of indirectly force Opera to switch from their amazing Presto, which is able to withstand 120 tabs opening, to WebKit, which can only handle up to 20 tabs before crashing.
Fact: Presto engine is the best engine, WebKit is terrible.