Voltage Generator with Full Wave Rectifier
You are driving your simulated bridge with two sources ​that are in-phase. Presumably, these are supposed to represent the output of a center-tapped transformer. This is incorrect &mdash; the sources should be out of phase. Either reverse the connections to the lower source, or change the sign of its amplitude parameter.

With them in-phase, they both reach their positive peak during the first half-cycle, and their negative peak during the second half-cycle, which explains your results. This is why you should always graph both the inputs as well as the outputs in a simulation &mdash; this issue would have been immediately obvious.