What caused my TNT Minecarts to explode?

Question

So I'm planning a violent execution:

I'm planning to set up a load of TNT carts around the convicted, and ignite them all at once by activating the rails.

So far so good...

The carts have started jostling, but no problem, I can fit more on.

Wat? I was just blasted a few chunks away as all the carts exploded at once. The rails weren't powered.

The wiki only has the following to say:

When given a redstone signal, they turn on and will activate any cart that goes over them. When off, they act as normal rails.

and on the carts themselves:

It will detonate on these conditions:

• It moves over a powered Activator Rail (after 4 second delay like the TNT block).
• It derails and falls more than three blocks.
• It's destroyed while in motion.
• It's destroyed by fire, lava or an explosion.
• It turns on a curved track with a solid block or entity located beside the track (in the previous movement direction).

What then caused my carts to explode?

Answer 1

It would appear that if 2 TNT minecarts run into each other on a curve with sufficient speed, it will set them off.

Step 1: Build a loop.

Step 2: See if Minecarts explode when place next to each other.

Nope, looks like this is safe.

Step 3: See if Minecarts explode when across from each other.

Nope, this seems fine too.

Step 4: Try to make minecarts explode on straight sections.

This never set it off (this picture is different because I tried many times to figure out where it was exploding).

Step 5: Make minecarts crash on a curve.

Ah ha! This resulted in an explosion.

This did not happen every time the minecarts collided in the corner, so this seems to be somewhat randomly caused.

Further Science

Does it need to be 2 TNT carts?

No! In fact, regular Minecarts seemed to set off the TNT more frequently.

Also, chickens are friends, not food fireworks.