There is no “default” ordering that a query will fall back on outside of an ORDER BY clause.
- Results may come back in the order of the clustered index, or they may not
- Even if results come back in the order of the clustered index on one run of the query, they may not come back in the same order if you run it again
If you need results to come back in a specific order, you must be explicit about it in the ORDER BY clause of the query.
Click through for a demo proving these two points.