Lack of oil stretches them quickly too as well as lots of kickback and brake engagement. I've seen some guys who have a habit of hitting the brake after every cut, but before the chain stops, and by the end of a tank of gas the chain is sagging off the bar.
Chains have been known to break whenever.
If a new chain starts out with the tensioner most of the way out, I either use a bigger sprocket, or 1 less drive link, which means that sometimes I have to assemble the bar, chain, and sprocket before installing it as one unit on the power head. and let that chain just live on that saw until it stretches enough to come off. I had a 24" archer bar that was too long to let a standard 84dl chain go on with the rim already on the saw. Also a 92dl chain that was 1 link shorter than I needed, but I could barely get the rim on, then slide it all onto the clutch drum.