Other mistake: In the scene where Duncan has been captured by the Watchers and restrained in the upright table, take note of the needle. The camera pans around the Watcher's head and then shows the syringe delivering its contents. Once the dose is fully administered, it can clearly be seen that there is no needle on the end when he holds it down near the Highlander's chest.
Other mistake: As Kate/Faith is walking to meet Duncan, there are cars parked at the side of the road,the scene is meant to be in London but the number plates are the wrong style for the UK.
Answer: It seems some catastrophic event occurs. This movie follows the continuity of the television show. In one episode Joe Dawson tells Duncan MacLeod the last time it happened was when two immortals fought in a temple at the base of a volcano. He tells him this caused the eruption that consumed Pompeii. But, Dawson also specifies that it's only a legend, and can't be confirmed as actually being THE specific reason for Vesuvius's eruption.