@Phantom Rider  
The painfully funny prainbow.

Taste the PAINbow!

It’s animated in a way that it implies Discord wasn’t watching during the rainbow attack…
 
But I like to think the rainbow rushing at him is what he found so ‘hilarious’.

@Alara Rogers  
Very good theory you got there! I like it!

My theory is that he knew what the Tree of Harmony was and what it was supposed to do – the plunder seeds were intended to kill it, after all. But the Elements of Harmony hadn’t been physical objects before that – they were manifested by the tree for Celestia and Luna, the same way the Tree manifested the box with the Rainbow Power for the Mane 6. So when the two alicorns pulled out the Elements, Discord laughed hysterically because he’d been expecting an attack from the Tree of Harmony, and instead here they come with pretty magic gemstones. He didn’t realize that the pretty magic gemstones were in fact the fruits of the tree.

@Amethyst_Crystal  
Considering he had a plan and didn’t care about being turned to stone, hell, he probably chose that pose ’cause it seemed appropriately silly at the time.

@Amethyst_Crystal  
No kidding. He was completely unaware of how he was staring his first defeat right in the face, and was not even worried.
 
Also, Celestia and Luna’s magic auras before they touched horns and their auras became a single, white aura from the Elements of Harmony: Colors of the University of Michigan, baby!
 
Blue and Yellow! WOLVERINES!! SO AWESOME!!!!

@Beau Skunky  
Yeah, I thought so too! I figured he was mocking something or someone.  
@PonyPon  
Instead:  
Laughing his ass off because he thinks so little of Celestia and Luna’s defiance of his rule, because he believes himself to be absolutely invincible and impossible to defeat.

@PonyPon  
Well, we still got an amusing Discord song this season, so I can’t complain.  
Water plz.

@Beau Skunky  
Yeah, what a letdown.

Y’know, I always used to think he was in that pose, ’cause he was singing.

Discord never saw it coming.

when the horns touch the magic happens.