Like Scottyb said, it could have been a shorted cell. But it might have been a corrosion within the connector, or some combination thereof and maybe other factors.
If you look at the gap between the copper connector and the insulation in picture #2, you'll see green corrosion covering the wires.
To avoid this problem (more correctly - delay this problem), use closed end lugs and shrink wrap the gap to make it air tight.
To each their own, but I prefer silver soldered lugs.
I got better low end performance when I upgraded to 2Ga cables on my 2008 PDS, but no top speed increase because the controller limits motor RPM.
When you get your new batteries, be sure top break them in properly.
Better hold on to your hat after replacing 8 year old battery and corroded cables.
