Cheap jumper cables would be my first guess. A lot of those cables you get at Walmart or in those little emergency kits you’re supposed to give teenagers at Christmas are garbage, and simply can’t handle the output of a good battery.
A good jumper cable should use a large-gauge cable. They should look like they’re for energizing Frankenstein. Mine are 0 gauge. I usually recommend a minimum of 6-gauge wire for very light duty stuff.
The crazy part is that the good ones don’t even cost much more than the junk ones. You can get a good set of 2-gauge cables for 35 bucks.