Let me guess here.

Following what appears to be current airline practice worldwide, to save on the fuel load, they probably had originally filed a flight plan to Tianjin (or some even closer location) with Beijing as an alternate.

So that when weather in Beijing was problematic, they could no longer afford to circle but had to land somewhere.

So far the practice has no led to any serious accident, so maybe it is actually OK. However this type of unpleasantness is a by-product.

Incidentally, this is not specific to AC.