Excerpt from this article:
Since 2010, the default avatar on Twitter has been an egg. The idea apparently was that a new user was like a gestating bird, soon to make its first tweet. It was designed to be playful and cute.
But over time, Twitter’s eggs came to symbolize something different: users who remain shadowy on purpose, to harass their fellow tweeters.
Today, Twitter announced that it was doing away with the egg as its default avatar, opting instead for a nondescript person-shape figure. No more bright colors, either — the new avatar is all gray.