Board Thread:General Discussion/@comment-32644080-20180921001549/@comment-28278649-20180921022756

The 'bill coming due' was in response to them using magic to tamper with the natural law, i.e. reversing the destruction caused on such a scale by using time magic. Mordo likely had no idea Thanos sought the Stones, as Wong didn't know either. Wong merely commented on the increasing appearance of the Infinity Stones, which Masters likely knew about since the Eye contained one.

Mordo was likely referring to some sort of mystical karma, which he was acting upon by removing the number of sorcerers using magic. He sees it as unnatural, thus setting up a potential sequel