What force makes fluid flow faster in a smaller pipe?

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Thinking of individual fluid particles, my guess is that the "force" is simply particles backing up somewhat at the constriction point and contributing a higher proportion of collisions on particles ahead of them in a direction down the pipe. As the average movement of the particles is directed more and more parallel to the pipe, fewer particles impact with the pipe itself, resulting in lower pressure on the pipe.

I see lots of answers about the kinetic energy increasing, but I can't see where that energy comes from... To me it seems like the total energy of the particles stays the same.

Put another way, Bernoulli's equation shows the relationship between kinetic energy and pressure, but what causes the change in pressure or kinetic energy?

 

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Whoops, reading it I realized I didn't say quite what I meant - not that fewer particles collide with the pipe, but that the angles of the collision become smaller and smaller.