It doesn't matter how new the tire is or how good the wet performance and sipes are. Typically the road surface is the most slippery in the first 30-60 minutes of rain. Why? Because when the rain falls on the road, it pulls the oils from the road up to the surface which makes it extremely slick. After a certain amount of time, the rain water can wash the majority of the oils off of the road which would reduce the chances of slippage. Right at the beginning though, you can expect it with the best tires in the world even.