I dunno totally about this, but I would predict that the slalom causes the fluid to slosh back and forth exposing the pump intake. The pump then sucks in a small amount of air that gets pushed through the lines.. Air+Oil = Foam under pressure. I'll just wager that the foam continues to expand until it pushes out the fill tube, or worse yet, fractures the plastic lock tabs and blows the entire cap off.
If I'm correct, then something that allows the pressure to bleed off and capture the fluid/foam might fix it up. Something as simple as an external tank maybe? Maybe something similar to the coolant overflow bottle. Something about 2x the fluid capacity might work.
Something else that I think could help is to over fill the reservoir initially so that the pump won't cavitate as quickly.
I think I'll try something like it on my black car. Thanks for the heads up.