It's related to our CDN static content distribution system and cross-domain requests handling, which is a way of distributing visitor requests for certain non-changing ECF content plus content supplied by others, without lags caused by network issues. In theory, anyway...
Because of a combination of vBulletin software code errors and implementation issues, it does not work faultlessly. For some people in some locations, there is a temporary network crash that means pages won't load without the locally-provided content. This is primarily a forum software error because the page requested should load the ECF content regardless of whatever else is requested; there is also a CDN error in order to cause the issue to arise in the first place.
So it's a chain error caused by at least two faults and only affects certain visitors occasionally. To fix it we'd need to move to more modern forum software (vBulletin is no longer properly maintained), and double our server cluster size so that all content can be hosted locally. It's a possibility, but not a priority, because the current system works OK for 99.99% of people, 99.99% of the time.