Every spring in Japan, cherry blossoms bloom, giving the country a pink hue. This is why pink is seen as a spring color. It's also why this McDonald's hamburger has pink buns.
In the past, McDonald's has released special sakura, or "cherry blossom," teriyaki egg burgers that are slathered in a special sakura sauce (a pink mayo with Japanese radish spread). These are limited time only meals for spring.
As pointed out by Japanese food site Entabe, there's a new twist for this spring's burger: a pink bun. Via Net Lab, here's a comparison to the other teriyaki egg burgers: