Suppose the market just tracks widget factories. Every factory produces the same quality widgets, all shipping is flat-rate by distance and there are no tariffs or taxes. In this market, it's easy to determine market value of any particular widget factory: market share of world-wide production.
Suppose everyone expects the demand for widgets to grow by 1% per year for the next 10 years. This growth might get priced into the market with some sort of net present value calculation. Capital will flow into building new widget factories to fund all of this new construction.
Now what if actually an asteroid hits a city with lots of widget demand. 50% of demand is wiped out instantly. Well in this case, everyone was overvaluing the market for widgets! All of those net-present-value calculations were way off. The market will correct.
Or maybe the asteroid takes out 50% of production. Suddenly much more investment in widget factories is required. The price shoots up to capitalize all of the construction, since demand has not changed.
In either case, before the asteroid the market was not correctly valued since no one had yet priced in the asteroid.
Who is it overvalued by, and how is the "true" value determined?