It's tugging at the hose and that's causing the pre-filter or intake end to be jumping around, which in the field would be causing it to hop around on the bottom or up to the top, stirring up sediment or even sucking air.
This is the first I'm hearing about this. I can see how it would cause a concern. My own personal experience has not resulted in me noticing this when demonstrating it in clinics. I think it takes a little getting used to just like any new piece of gear. One thing I have noticed is that the prefilter doesn't need to be fully submersed in order for it to work. The hose is also pretty long and the velcro attachment can be used to anchor down the prefilter.
I haven't used a production unit in the field (I've been using a prototype that hasn't been approved to be used in the backcountry) yet and as soon as I do I'll let you know if it is an annoyance.