A Strangelet can only happen inside neutron stars, where gravity keeps the Weak Nuclear force under control, preventing Strange quarks from degenerating into lighter quarks. In a neutron star, Strange quarks are appearing and disappearing all of the time, but it's only when the neutron star itself is massive enough that it creates a runaway Strange matter conversion process. The reason this happens is because Strange quarks are heavier versions of Down quarks, and it allows a neutron star to get heavier without increasing its diameter. In fact, the neutron star will decrease its diameter, since a Strange quark is between 17 and 25 times as massive as a Down, which will make the star even denser.
In a particle accelerator, Strange quarks are created all of the time, but no runaway chain reaction of strangelet creation happens, as there is no equivalent source of gravity on Earth.