These extreme electroweak stars will "burn" quarks, converting them into smaller particles called leptons, generating massive quantities of energy, flooding space with neutrinos (extremely weakly interacting particles that pass through matter like ghosts). The quark energy released would create an outward pressure preventing the force of gravity from pulling all the matter into a single point (i.e. a black hole).
Under such extreme conditions, the electromagnetic and weak forces effectively become one single force, hence the name "electroweak." The only other time we think such an extreme state of matter existed was moments after the Big Bang, just before the electromagnetic and weak force became the separate fundamental forces see measure today.
Neutron stars resist collapsing under their own gravitational pull by a characteristic of matter known as neutron degeneracy. This produces an outward force called neutron degeneracy pressure.
What if the neutron star born after a supernova is too massive for this neutron degeneracy pressure to hold up against the neutron star's own gravity? In this case, it's up to the quarks that make up the neutrons to take over, preventing the body from collapsing any further. Single neutrons are composed of three quarks (two "down" quarks and one "up" quark).
When quark degeneracy pressure kicks in, a quark star may be produced; the free "up" and "down" quarks get converted into "strange" quarks. Therefore, a quark star is made up of strange matter.