My bet is it is cause by drilling or tunneling, the museum is situated in the grounds of Manchester university* and next to a busy road (Oxford road).
I would like to know the exact weight of the statue because I have read that it is made from "steatite" aka "soapstone". my point being the lower the weight the less vibration needed to move it.
although the statue in question has a flat base, it is carved in the classical Egyptian style with the left leg forward, suggesting to me that if vibration was applied from below it would cause the rotation seen in the video.
after a bit of searching I found this at the museums own website
"So we can make some improvements to our
Ancient Worlds galleries, parts of Ancient Worlds will be closed on the following dates:
Mon 15 Apr-Wed 1 May 2013: Discovering Archaeology gallery will be closed*"
The time/date code in the video reads 2013/04/03 which I think is the 3rd of April.
in a nutshell I think there was "construction" work going on, probably involving drilling or grinding in the same building.
one other thing I noticed from the article is where it says:
"
But it has been on those surfaces since we have had it and it has never moved before,"
The cabinet in question looks far more modern in style to one built in the 1930s(when the statue was acquired by the museum!) so the idea that "
it has been on those surfaces since we have had it" is clearly wrong.
also construction work would also explain why it only happened in the day time.
*((source))
Ancient Worlds (The University of Manchester)