Weak link in the chain

Two days ago I installed a new chain on my bike. Regular maintenance, it was working fine but starting to stretch. I figured a new one before the big ride was only prudent. I changed it, went for a 10 km ride, all was good. I was happy.

The next day I rode over to ICBC and back - about a 2km round trip. On the way back the chain broke. I can't recall ever breaking a chain before. Luckily it was a short walk back to the house, and even luckier, I had not yet thrown out the spare links from my new chain.

I am not 100% sure why the chain broke, but I have suspicions. I connected the chain correctly but as you can see in the photo the link appears to be stretched. That would certainly explain the pin working it's way out. So I am not sure if I picked up a crappy chain, did something wrong or just had a bad link. I was able to replace the broken parts and things seem to be working ok now but the new join does seem a bit stiff. It's only noticeable when the stiff link goes over the reverse cog on the derailleur though, and only under no load (ie. pedaling backwards while coasting).

Needless to say, I'm packing my chain tool and the rest of my spare links to take with me on all my rides for the next while.