It was done to update the servers and software, you can read the detail here. https://discuss.flarum.org/d/7585-free-flarum-hosting-on-an-expert-platform-by-freeflarum-com/3882
I didn’t notice the update about downtime today, so it came as a surprise to me. Always remember, bookmark this page: https://status.freeflarum.com/ and use it to find out what’s happening if the forum is down, you can also view the dicussion thread link above.