There are two reasons I can think of that this may happen.

1.) As you reach the end of a file, segments start becoming completed. When a segment copletes, the conenction is dropped and new segment sizes are caculated. Hosts that you were just connected to might become busy as soon as you discnonnect, leaving only a couple remaining segments downloading with no more open hosts to connect to. There is nothing you can do about this.

2.) When downloading a very popular file, occasionally you download from someone with a corrupted version. If the 2K data buffers don't line up correctly between segments, the download is dropped and deemed a 'fixed segment' when XoloX reaches 10 of these it goes into a 'safe mode' where it only downloads from one host at a time. You can tell XoloX not to do this by unchecking the appropriate box on the Downloads tab in your preferences.