The Minoan civilization is the first civilization in Europe, and it is deeply admired and lot of people are fascinated by it, as it was a very advanced civilization for its time. The decline of it is not clear yet, and there are multiple theories about it. Some of the theories include strong volcanic eruption, strong earthquake, a tsunami, and invasion.
As a most plausible reason for the decline of the Minoans I would consider a mixture of a strong earthquake and invasion. The Minoans lived at a place which is geologically very active, so the earthquakes are common occurrence. A strong earthquake is very possible to have happened, and considering the period of time, it would have been devastating for this civilization, as the towns would have been destroyed totally, lot of people would have ended up inured and dead. The economy and military strength would crumble because of such an event, so an outside invasion would have been very easy. The weaken Minoans would not have been able to defend themselves, so it would have been an easy and attractive place for invasion.