Infested blocks can be destroyed immidiately when in survival mode

When I was in the stronghold and I used an unenchanted diamond pickaxe to mine an obsidian, I accidentally mined on an infested block and it immediately spawned a sillverfish. That happens also when I am using a shovel. I mined on an infested block and it still broke immediately in a tick and it came out silverfish. How can that happen? I am confused.