If you think about it, the knifefish use electrical sensory to navigate.. When you put that syphon in the water it probably creates new currents and vibrations that it can sense.. It's structure could be similiar to a rival knifefish species or some other fish.
I have had a large Carapo knives come out to 'investigate' syphon tubes, but seldom did they attack it.. Sometimes they would try to swim into it, some would try to cuddle with it. Interesting i would say.
That is a nice knife, How big is it? It has quite the stripe!! I only thought the brown ones got those kind of stripes?