Well, Vel'Koz seeks knowledge, he's the Eye of the Void. His entire purpose is to learn infinitely. Karma is the Enlightened one, implying that she has vast amounts of knowledge. Vel'Koz would naturally want it. That might not make sense, but that's my idea of why it's this pairing.
It's a little more basic than fluff. Both have a color scheme involving lots of purple. Both have beam attacks. Both have AoE delay slows.
But one is support, the other is damage. One is concerned only with himself, while the other can't stop thinking about the many. They are similar, yet opposite. And opposites attract. Even more so when they are also similar.
