In a 'wooded_mountains' biome, just above bedrock level. I'm breaking placed torches to pick them back up and I am noticing that sometimes the block beneath (which I'm assuming was a monster egg block anyway) is spontaneously breaking and releasing its silverfish.
Environment
Win 10, Java 1.8.0_51 64bit
In 17w47a (a snapshot for 1.13), monster eggs got renamed to infested blocks and now instantly break. You may have accidentally clicked on the infested block.