The reference to dbo.SORT in the error message says to me that the growth was due to an internal work table that spilled to tempdb. Some reasons why this could have occurred:

- It could have spilled due to memory pressure that is not always present.

- It could happen if an ad hoc query returned more rows than the query author expected (cartesian product maybe) and the execution plan had a Sort operator within it.

- It could happen to a query that typically runs normally if your stats are outdated and the engine picked a bad plan with a Sort in it that it may not have picked had your stats been up to date.