I wonder what the reasons behind this are?

Less opportunities going around?

More competition for the same number of opportunities (from other funds or cashed up companies such as FB & Google buying out companies early)?

A reaction to the "super angels" and their successes?

And the fact that their investment in YC itself is kind of a Sequoia "Ycombinator" of sorts (I am sure there's a way to derive that using lamdba calculus :-)