Glad it's working 
I'd also say it's likely the file was getting corrupt somehow -- whether by transit, a program, or the storage medium it was being saved to.
There is a way to check a download is bit-by-bit perfect by performing a SHA256 checksum. The download page lists one like this:
dc3afcad68a5de3ba683dc30d2093a3b5b3cd6b2c16c0b5de8d50fede78f75c2
If you're using Mac/Linux, you can do that right in from the terminal:
sha256sum ubuntu-mate-16.04.2-desktop-armhf-raspberry-pi.img.xz
They should match - and should help identify corrupt downloads in future. 