Why are 1989 Score Troy Aikman PSA 10 cards experiencing such a jump in value?

[sjm76]

I bought a PSA 10 1989 Score Aikman about a month ago for $140 and a month later they are selling for between $400 and $600. Not that I'm complaining but why such a jump in value? I'm thinking that it's because other PSA 10's in the set for stars like Barry Sanders and Deion Sanders are selling for around $300-$400 but is there something else I'm missing?