As said when laying Spiketraps manually under the Riftguardian nothing happens. Even the Riftguardian porting into them wont make them go off and do some damage
Edit : i use Lightning Rod and Scatter (from TragOuls Bracers) Rune
What rune are you using?
If you are using the Custom Trigger rune, they do not detonate automatically.
I edited my post : scatter plus lightning rod as runes for the traps