newtonian mechanics - Is a causal relationship implied by Newton's 2nd Law?

Throughout my time learning physics I have been imbued with the notion that forces cause accelerations, period. Accelerations don't cause forces, and they aren't merely correlated phenomena. By causality, I am content with the following definition:

Connection between two events or states such that one produces or brings about the other; where one is the cause and the other its effect.

That is to say, an object experiences an acceleration because it is exposed to a net force; the force does not arise because of the acceleration. However, some philosophical thinking on Venturis has shaken my confidence in this idea. If the acceleration of the fluid through the constriction is caused by an unbalanced force, what causes the unbalanced force in the first place? Another way of asking the question is, how is the bounding geometry causally linked to the pressure distribution of the flow? My only answer as yet is that there's no other way to satisfy mass, momentum, and energy conservation simultaneously, but that seems decidedly unsatisfying. Is there any causality implied by Newton's 2nd Law?

Answer

One of my favorite quotes, and I think this complements Ján Lalinský's answer:

"Does the engineer ever predict the acceleration of a given body from a knowledge of its mass and of the forces acting upon it? Of course. Does the chemist ever measure the mass of an atom by measuring its acceleration in a given field of force? Yes. Does the physicist ever determine the strength of a field by measuring the acceleration of a known mass in that field? Certainly. Why then, should any one of these roles be singled out as the role of Newton's second law of motion? The fact is that it has a variety of roles." - Brian Ellis, The Origin and Nature of Newton's Laws of Motion (1961), as cited by A P French, Newtonian Mechanics. (a fantastic book)

$F=m\ddot{x}$ isn't a definition of force or a definition of mass, it's a relationship.

As for your specific example, I can't help on the dynamics, but from the fact that the water accelerates, it must be pushed from behind (or pulled from the front, but you know with pressure the two are equivalent). This image on wikipedia:

http://en.wikipedia.org/wiki/File:Venturi.gif

in which high pressure is indicated by a dark blue color, gives you a pressure gradient. Clearly the change in pressure is enough to explain the acceleration/deceleration. Why is there a pressure gradient? That is deserving of its own question*.

*I'm answering the question "Is a causal relationship implied by Newton's 2nd Law?"