randomly spawned monster egg blocks

Was playing survival and was digging around in a mountain. I dug a block I thought was a normal stone but a silverfish popped out, I hit it and 5-6 more silverfish came out from other blocks. This happened at two or three different places in the same extreme hills biome. I thought it was a stronghold nearby that spawned the silverfish, but I checked in creative and there was no stronghold nearby. I am not certain this is a bug, but I couldn't find anywhere that monster egg blocks were supposed to spawn anywhere but in strongholds, and I have never had this happening before.

Seed:519811158758600062
The coordinates for a patch of seven monster blocks: x:-808 y:46 z:473