slip n slide

A little background on my situation.
I have yakima round bars on my car at the moment. After a 5hr ride on the San Joaquin river I load up my Hobie mirage and head home. (I always strap my kayak upside down and tie it at the front of the car) Just about 2 miles from my house at a stop light I noticed my kayak moved forward a little. I thought to myself, "maybe my eyes are playing tricks on me. It has been a long and exhausting day." Thinking nothing of it I accelerate and suddenly my kayak slides back. Thank goodness for the front strap. I immediately pull over and tighten up the straps, but everything was as tight as it was when I left.

Could it be the moisture/water from the river causing my yak to slide?  What can I do to prevent this in the future?
Are the yakima cradles any good?
I'm  paranoid to transport my yak now. 
 (This never happened when I had my Subaru with the factory rack)
Any thoughts or ideas will be greatly appreciated.

We're the tie downs close and equidistant to the kayak? Are the straps wide and high quality?

Put some pool noodles on the bars. The noodles will prevent the slick plastic of the kayak from sliding on the slick plastic of the bars. The compression of the foam maintains the pressure from the straps and stops the sliding.  Even with temperature changes, the straps will stay locked tight.

Thanks fellow kayakers. I will give the pool noodles a try and my lashing straps are made by yakima as well. I was thinking maybe low surface tension due to the round bars.

Hobie Mirage?  I always run a third strap (or if parked loaded for a while, a bike lock) thru the drive well and around the rack bar. 

Hobie Mirage?  I always run a third strap (or if parked loaded for a while, a bike lock) thru the drive well and around the rack bar.

This....it won't be going anywhere.

Put some pool noodles on the bars. The noodles will prevent the slick plastic of the kayak from sliding on the slick plastic of the bars. The compression of the foam maintains the pressure from the straps and stops the sliding.  Even with temperature changes, the straps will stay locked tight.

Exactly how I do it and haven't had any problems. and I always use front and rear tie downs.