I haven't experienced down time when I've used the forum. Each time I open the site the forum's available there aren't issues. If it's getting problems on Sunday morning it's happening at times when I'm usually not online. I'm glad I didn't see it.
I'm going to bet it was just some server glitch or a server migration downtime by whatever server service Theymos is using. The server I personally use — Vultr, frequently has maintenance but little to no downtime.
Correct me if I'm wrong, but is there even a reason for anyone to DDoS Bitcointalk knowing that there is no other close competitor anyway? Lol.