What do the volume increases in certain trend lines signify?

For example when looking at a triangle pattern, rectangle, or whatever it may be, when the price of the stock breaks through the trendline indicating the direction in which it should/could go, there is supposed to be accompanying that breakthrough an increase in volume (while before that increase is supposed to be a steady decrease). What does that increase in volume represent? I am just trying to understand what it is that causes such an increase at that point?