The rebound slide spring not only performs the essential task of resetting the action and returning the trigger and other components to their "stand by" positions, but it also holds necessary tension on the trigger bevel when the action is cocked SA.
The trigger bevel is the very sharp, angled edge at the top of the trigger that interfaces with the SA cocking notch on the hammer. When the hammer is cocked SA, the tension from the rebound spring, working together with the pressure exerted by the mainspring, keeps the point of contact (bevel) standing at rest under tension on an approx .008" wide ledge called the cocking notch, or SA notch.

Decreasing the stardard length (OEM) rebound spring tension does not in and of itself cause push off, but merely lowers the threshold where the push off can occur. All other things being equal, it makes the interface less viable, if you will, when viability is evaluated purely in terms of SA trigger pull weight.
Other factors also negatively effect the viability of this interface, including dirt and debris, unsuitable lubricants, wear and tear, modifications, etc.

It's important to consider the role of the rebound spring not only in terms of it's job in trigger return, but also the important role in SA integrity as well.

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Ret. LE, FA Instr, S&W Armorer