I've been told that you're not supposed to use insulated terminals on ranges, and the reason given is that the heat they're exposed to in service might get that plastic insulation burning, and that fire could then spread to the insulation around the oven, and that larger fire could then spread to your house. I really don't know how much of a concern it is since I've use insulated terminals on stoves many times, but I only use them when connecting wires in relatively cool areas, not on the back of the bake or broil elements.
There's something called the "million dollar crimp" that an ex-appliance repairman taught me about. When you crimp a terminal onto a wire, you should always crimp it on as tight as you can. If you crimp it on so that it doesn't come off, but isn't tight, then that terminal will burn off in a year or two, exactly as you've experienced. So, an unscrupulous appliance repairman could keep himself busy by having lots of terminals burn off by not crimping them on hard enough.
The reason that happens is that when the terminal gets hot (due to current flowing through it, or due to conduction from the bake or broil element) it expands, and if it's not on tight enough to begin with, you can have a very tiny air gap open up inside the terminal, and arcing between the terminal and the wire it's crimped on to. That arcing causes corrosion inside the terminal and on the wire, and that ends up causing the terminal to get hotter than it would other wise be. The hotter terminal expands more, causing more arcing and more overheating. And so you effectively end up with a snowball rolling down hill. The terminal keeps getting hotter because more corrosion forms between it and the wire. Eventually the wire just burns off at the crimped terminal and you need to cut the burnt end off the wire and crimp on a new terminal.
If you have a small pair of side cutters, you can just cut the plastic insulator off the insulated terminal you used. I noticed that you used a yellow terminal. Insulated terminals are colour coded with red insulation being for 18 to 22 gauge wire, blue insulation being for 14 and 16 gauge wire, and yellow insulation being for 10 and 12 gauge wire.
This is a guage for checking the American Wire Guage (or "AWG) of wires. The larger the guage of the wire, the smaller it's diameter.
I think you just got a poorly crimped terminal. That can happen in any appliance, but stoves are the worst for it because of the large amperages going through the wires. I wouldn't worry about it unless you get a rash of terminals burning off. The best way to repair a burnt off terminal is to SOLDER the terminal onto the end of the wire, but if you put an honest effort into squeezing the bygeezus out of your crimping pliers when crimping the terminal, that'll last a very long time, too.
And finally, there will be TWO fuses or circuit breakers (each 40 or 50 amp) going to your range. You need to remove BOTH fuses or trip both circuit breakers to cut off all electric power going to your stove. You need to shut off all power to the stove to work on it safely.