FU Orionis variables are protostars that go through massive outbursts that drastically change the star's spectral type and magnitude. With the star V1057 Cygni, this star was known to go from a K type dwarf star (K V) to an F-type supergiant, according to Wikipedia. In 2016, a study showed that this star had a companion. So does this change any property (age, reason for eruption) of this star, and why did this FUor have such a massive change in luminosity?
FU Orionis episodes (for that is what they are, rather than a particular 'type' of star) happen due to large infall of material from the circumstellar disc. They appear to be a normal stage in the formation of low-mass stars. We know of so few FUORs because the FU Ori episodes happen on widely-spaced timescales of several thousand, or several million, years. So it's not that they are especially rare - it's just that we don't live long enough to see many!