Locks caused by async stats update
It has nothing to do with SCH-S. Select queries which acquire S and IS locks are compatible with SCH-S, which is called Schema Stability and all it does, it prevents table during query execution from being modified which means SCH-S and SCH-M are not compatible. SCH-S is compatible with all other locks.

Your problem however is that you have auto update stats enabled, which are triggered whenever there is certain amount of data changed within a table (20% pre SQL2016 or with 2371 traceflag counts % depending on number of rows) and it does when you execute the query. 

So what might have happened is that your table was pretty busy and query optimizer decided it should update statistics, so you had to wait. 

You should disable auto update statistics and update it manually