With original credit to Doorknob for providing the picture, I can confirm that the heart shape is indeed real (unlike the chickens, those were a troll by Telkitty).

[![enter image description here][1]][1]

Thanks to THelper suggestion, I can confirm that the trigger seem to be related to clicking (on the head?) of the stick figures: doing that, I was able to trigger the heart formation multiple times.  
When the stickmen start forming the heart, you will notice they suddenly stop fighting each other.

I think to also know why this egg was added.
When the countdown to winterbash started, the site just contained a dark background, with the stickmen running around to form the numbers of a countdown.  
Then, one use posted this question: [How do I interact with people?](http://meta.stackexchange.com/questions/270223/how-do-i-interact-with-people).

> I'm afraid I'm a little shy, and it's hard for me to find the right way to interact with the people.

> That said, I've tried several things, and nothing is working.

> For instance, when I click on one of them, they don't seem to notice. Waving the cursor over them doesn't alter their behavior either. Clicking and dragging yields no response.

I bet that balpha took the idea from there and decided to add something that would be triggered if someone tried to interact with these small ones. 
  
*... now, why he didn't take my suggestion to replace those humans with some unicorns I don't know...*




  [1]: https://i.sstatic.net/Omrhr.png