When you pickup a (big) stone, sometimes spiders emerge from under it. They can trigger fly swatter traps (havent tested the other traps but I assume the same applies).
#Traps are activated by spiders coming from underneath stones
tardy totem
upbeat flame
Spiders got physics, you can also kill them
tardy totem
Spiders got physics, you can also kill them
I know. In the recent patch they made sure that small critters would not trigger traps anymore, so why would the spiders be any different.