Not an answer but we are seeing the exact same peak! We have been trying to figure out what this is for weeks.. Did you ever find an answer? It would be a GREAT help.
I believe this is called peak fronting. It's a electrophoretic artifact, due to a stacking effect of DNA fragments that have similar mobility and stack together, creating their "own" peak. Peak fronting typically does not alter results significantly, as long as the samples are meeting kit sizing and concentration specifications.