If the contractor got trash in the mainline while the valve was being replaced it could stick a valve open. This would happen right away. If a contractor replaced a valve he or she should have checked the entire system to make sure that none of the valves stuck open. I agree that coincidences are not that common in irrigation repair. The valves that aren't turning on or are irregular may be a separate issue or while the contractor was working on the other valve he or she may have cut or loosened the connection of the cable that go to the rest of the system and that could account for the irregular operation.
Good Luck