There are two major failure modes in startups. The first common failure mode is the thing you make doesn’t get adopted. That’s called not finding product market fit in startup lingo. The second common failure mode is “getting too far out over your skis” and it happens to companies that do find product market fit but mess things up by building an inappropriate cost structure (and capital base) and it all comes crashing down on them when they either can’t continue to raise money at ever increasing valuations and/or when they can’t grow into their cost structure quickly enough.
Source: The Finance To Value Framework