The Russian problem is that they have a limited number of ports due to their starting position. Since they're often at war with multiple people, the number of ports they have changes regularly as they take and lose provinces. Whenever the Russians cancelled a trade agreement with me, it was because they had just lost one of their trade ports and no longer had enough sea trade routes to support their existing agreements. I think the cancelled trade route that results is more of an automatic game thing, but it shows up as a 'Russian' action for some reason.