The word "trend" can be defined as the direction "something" seems to be going. This "something" can be animate, such as a flock of geese moving generally northward during spring in the northern hemisphere. Or the "something" can be inanimate, such as a rock slide or an automobile rolling to a stop. And then the "something" can be sociological, such as the direction a political election is leaning or the way education is changing.
All of these point out that a trend is not absolute, but the odds are in favor of "something" moving in a particular direction.
Adding the -ing ending to the word "trend" emphasizes the process or movement aspect of a trend. A trend may point to a probable end, but trending deals with the process of getting there.