Is there any way to spot these annoyances?
They "explode" because you walked through them while wearing power armor. It's not like people are stuffing their sandbag barriers with cherry bombs. That would be kind of stupid, considering they are typically there for you to hide behind when someone is shooting at you.
By the way, you can also knock over those short posts at the Starlight Drive In. A few other things I can't remember off the top of my head as well. It's just one of those cool little extras that are in the game to make power armor feel more like a vehicle than an oversized t-shirt.