Possible mechanisms: The absence of a belt allowed the pressure to drop more radically at the end of the set. Time under the bar may have been longer. Dietary factors. Hard to say from one isolated instance. I have also noticed a tendency for the experts at 70sbig to use the belt with rank novices, a practice with which I disagree completely; it may be that you are using it prematurely, before you are good enough at lifting without it.