Unrest pathing is infamous for this.
How it happens is that you sit in the basement or MR and get sit aggro (not like the one they botched recently), through the floor - usually lurking mummies. This was a design flaw of the zone. Sometimes however the mummies would like others, which tied to the upstairs and created a larger train.
Unless you were experiencing this during the few days of 'sit aggro,' then it's most likely working as intended.. or unintended
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