Yeah and eventually forget about the old logo after it's been replaced everywhere. But for now it takes me a few extra seconds to find the App on my phone.

Do you know what causes them to change their logo? Any articles?

This is from their official blog.

https://slackhq.com/say-hello-new-logo

Looks awesome! I'm excited to try the API documentation tool.

FYI if you haven't clicked on the animation on homepage, do it...it's addicting