Was pedalling up steep hill on gravel road. No gear change. Just seemed to go mid push. Was very steep and out of saddle....had to push the remainder of way! 🙈
Well, only really one proximate cause for this: chain was weakened through wear and fatigue (it looks a bit old), you applied enough force to break it.
Likely that there is no single event that broke or weakened the chain.
Chain failure happens. Think of it a a rite of passage.
The chain could have been damaged earlier, during a poorly-executed shift. Or the chain and/or sprocket may have simply been worn out -- a worn chain/sprocket combo causes "chain suck" which can break the chain.
Worn chain: Check your chain wear regularly using a chain wear gauge. If the 1.0 side falls through it’s high time for a new chain. For me with 10-speed chains the 0.75 side usually falls through after about 2000km.
Bad installation. You should never re-use a normal chain pin. Only use a “master link”/“missing link” or a special closing pin.
Previous damage. Maybe you previously had a bad shift or bent the chain when uninstalling the back wheel etc. Try to be careful, chains are strong in the direction of their links but weak when twisted or loaded sideways.
I’ve never had a broken chain despite bicycling tens of thousands of kilometers, often through mud and with luggage under high power. Therefore I wouldn’t carry around a chain tool despite what others have suggested.
I personally haven't seen a metal failure in a chain plate.
Clean it up and have a look under a microscope at the plate to see what the fail edge looks like. (Your cellphone will take a photo through the microscope nicely)
It is hard for me to imagine a failure mode other than a manufacturing defect (crack) in the plate. The failure modes I am familiar with result in popping the plate off the pin. (e.g piece of gravel between chain and sprocket)