I don't own one of these--are there any conditions under which Alexa will laugh normally/in an expected way?

This falls under Furby bugs level of weirdness.

http://official-furby.wikia.com/wiki/Furby_Bugs

Alexa laughs creepily when you wish it a happy Halloween, I'm not sure if it will do it for any other reason.

Another comment lower down says it does this exact laugh if you say, "laugh for me".

So likely it's a false trigger on that phrase. It would be amusing if it's some kind of malware however.

This is terrifying: https://www.youtube.com/watch?v=owqemRL1BMA

I will be honest with you. Lately everyone's been grinding the word "terrifying" into the carpet with their boots. But this. I was simply not prepared for this. This was terrifying.

That page is fucking gold, thank you so much for sharing.

It feels too specifically creepy to be a random bug?

My guess is that a developer replaced the "I didn't hear what you said." response with this creepy laugh on his home Alexa, then somehow accidentally pushed that build to the public.

The other explanations are a hack or disgruntled employee which seem worse.

The command is "Alexa, laugh". Audio-wise, a mere cough could be interpreted as "laugh", that's why Amazon's solution is to change the command into something more pronounced.

Other unprompted activations have been reported in abundance when Amazon installed chatbot functionality after the Alexa Prize: Alexa would start having conversations with herself in the middle of the night. One can expect bugs for newly added features, and of course the creepy ones are more likely to make the news than any random glitch.

My Echo has triggered randomly when I'm nearbye and not talking. Only to say something to the effect of "Sorry I didn't understand what you say" so I'm guessing there is a bit of a bias in people reporting.

The updated article says that the phrase being triggered is "Alexa, laugh". As long as it somehow mishears Alexa, 'laugh' is a very short phrase that seems easy to accidentally detect (maybe even by a cough). That's probably why this one was occurring more frequently than other possible triggers, and would also explain Amazon's fix of changing the trigger phrase to "Alexa, can you laugh?".

I've never heard ours laugh in any context. Rarely it will trigger seemingly without provocation, but in that case it says it couldn't understand a question, or does something silly like define "fire". When it does that, we laugh at it!

>something silly like define "fire".

Could be silly. Could be a veiled threat.

Most likely selection bias at work here.