Probably because they were downloading from multiple hosts, but the one with the largest bandwidth to spare either went offline or you lost connection to them leaving you with probably downloadng partial downloads from other users. Else they are dialup users or else, a client like Shareaza (RAZA) which is reknown for having good movies but poor offerings of speed or else, miserly left overs from others in bandwith. Remember people come & go on/offline all the time. This can change how people are connected & who they are connected thru (as stated below.)

A change of Ultrapeers because one went offline may have also contributed.

Another reason might be your ISP. If this problem happens frequently at the same time then it looks like your ISP may shape p2p bandwith at peak hours of the day.