Sounds like a case for a group of meddling kids.

The video doesn't load for me. A similar article with video: https://news.ycombinator.com/item?id=5927510 (4 points, 5 hours ago, 3 comments)

Extract of comments: ... In case this is not a joke ... It happens only at daytime ... so it's probably dew to the vibrations caused by the public ... because the friction between stone and glass is small.

Note: An important detail is that it's not a "perfect circle", it's really only a half turn, so gravity is a possible explanation.

Another thread: https://news.ycombinator.com/item?id=5927620 (3 points, 4 ours ago, 4 comments)

As I said in the same previous thread, it's a smart advertising.

Wat.

I mean, seriously, WAT.

Most likely there is a logical explanation, but I'm curious how many people from HN read the article and immediately hope that it wasn't a scientific reason at all.