This happens because the way files are sent to you is in parts.

Say your connected to 14 people downloading, they are all sending you different parts of the file your downloading. So when it gets to about 90+% it will start dropping hosts because their isnt anything for them to send to you (the other hosts are already sending it too you).

It gets annoying because it usually drops the hosts that give fast speeds, but thats just the way it works. The best way to counter-act this, is by searching for what your downloading again, cancelling your download, re-clicking your download from the search and it will continue from where you cancelled it from, but it will most likely connect to more hosts this time becuase it will split the remaining parts of the file into smaller peices.

I hope you understand this - its like bittorrent.