There's a surge in games with strong team dynamics, for one thing. Fortnite is the most prominent example (it has the largest Discord server of any kind with over 180,000 users), but many of Discord's most popular verified servers are for games where team communication is essential, such as PUBG, Rainbow Six: Siege and Vainglory. Combine that with Discord's recent deal with eSports teams and the company is effectively the default choice for anyone who wants team voice chat and a place to discuss games in between matches.
And then there's the rise of livestreaming on services like Twitch, YouTube and Mixer. Discord has tight integration with some of these platforms, and it's extremely common for streamers to have their own Discord communities. Arguably, Twitch's recent support for Rooms (sub-chats inside a given channel) is a direct response to Discord's growing influence.
This isn't to say that the past year has been uniformly positive for Discord. The racist violence in Charlottesville highlighted the use of Discord by neo-Nazis and other "alt-right" hate groups, pushing Discord to crack down on users and servers that previously went untouched. While the company is no doubt happy to have grown as quickly as it has, it was also somewhat unprepared for the impact it could have beyond the gaming world.