FAQ: Why does ArcGIS Server lock the ArcSDE schema?
Why does ArcGIS Server lock the ArcSDE schema?
ArcGIS Server locks the ArcSDE schema because if the schema was allowed to change while the data is being used by other ArcGIS applications, other application would likely crash.
When an application accesses an object, many of the properties of the object are cached. This is why the data owner is prevented from changing properties when schema locks are present. The schema locking is done via table_locks.