* Disturbed existing leak: Working around the engine could have dislodged a small amount of dirt or debris that was previously plugging a minor existing oil leak. Now that the dirt is moved, the leak becomes more noticeable.
* Damaged something nearby: While unlikely, a dropped tool or careless movement could have nicked an oil line, valve cover gasket, or other oil-related component near the spark plugs. This is more probable if you were working in a cramped space.
* Over-tightening: While less likely with spark plugs, it's possible you over-tightened something else nearby (like a valve cover bolt) during the process, cracking it or damaging the gasket.
* Coincidence: The leak might have started independently of the plug and wire change. Perhaps an existing oil seal is finally failing, and the timing is just coincidental.
In short, the spark plug and wire replacement itself is highly improbable as the direct cause. The leak likely predates the work or was caused by an accidental mishap during the repair. You should carefully inspect the area around the spark plugs and wires for the source of the leak.
