|Maybe the youtube server you're downloading from is very busy. The busier a server is, the slower it is for everybody connected.|
Alternatively, the network could be very busy which has the same effect (the busier it is, the slower it gets for everyone).
If this is the only time you've experienced this issue, try downloading your video during non-peak internet hours and see if that doesn't improve the download rate.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.