Wow. I've never seen this before.
I was cutting up a leek the other day, and when I went to cut off the roots, my knife hit something hard. Like, as hard as a rock. I made a cut around the circumference and peeled off the tender part. Sure enough, straight up the middle of the leek was what I can only guess was a bolt stalk about an inch thick. You can see in the picture above there is a nick on the stalk on the right: that's where I banged it against the counter as hard as I could (out of curiosity). I could barely make a mark on it! I bought it at the grocery store too... it's not like it came out of our garden, or even the farmer's market. Very odd. I guess the moral here is to look into the top of the leek at the store (especially in the spring) to make sure you're not buying a bolted leek.