The non-technical explanation, in complete detail: "S*** happens!"
The technical explanation isn't much different. Server hiccups. MySQL glitches. Xmsn errors. Power dips. Flaky RAM. Disk errors....
A "page refresh" may yield the (apparent) result of clearing the error, but it doesn't have much to do with the underlying problem(s).
BTW, this is a complicated (by some standards) system. In a prior life, dealing with mechanical critters like airplanes, we frequently cited another truism: "Aircraft are complicated mechanical systems. Complicated systems break." With modest paraphasing, that applies here, too.