Well, just to put a unique spin on it--maybe it did fall over and was damaged.
Something to remember. A smaller Genesis wave--using the same proto-matter, carved out a paradise inside the planetoid right? If it were unstable, it should have been evident early on.
Here is my theory.
The device would have worked perfectly.
One, it was placed inside a nebula, where it had to strain in order to re-awaken the pulsar we see--converting degenerate matter back to hydrogen (most of a stars mass remains at its death). We saw that later in DS9 when Richard Kiley's character awoke a star with perhaps similar means.
Reliants destruction, and the device falling over changed the genesis wave. The planet died, but it allowed Spock to live.
I would say that, without being knocked over, the reverse would have been true.
This plays into my dream Star Trek Star Wars cross over (with Dr. Who thrown in)
The Doctor and the Enterprise bring Alderan back to life by firing the genesis torpedo at the Death Star instead.
an even swap--added mass coming from an asteroid belt.