Images went unable to be posted for several hours due to a "connection error". Posting text, however, worked just fine. A similar event occurred earlier last year, causing /x/ and /pol/ to explode, because the last image posted before the site went down was allegedly of an ayy lmao. As far as I know, however, there was no downtime this time.
So far Hiro has neither commented on his personal Twitter account nor on the official 4chan Twitter account about what caused this. There wasn't a sticky either.
On an unrelated note, post quality shot up, as did the amount of ASCII art. Presumably, because most phoneposters left in droves, since they tend to post shorter replies with images.