You're right to point out that the forces of pressure and gravity were in balance within the solar nebula. However, this balance was a delicate one, and there were factors that tipped the scales towards collapse, ultimately leading to the formation of the Sun and planets. Here's how it worked:

1. Initial Conditions:

* The nebula was not perfectly uniform: The solar nebula wasn't a perfectly uniform cloud of gas and dust. It had small density variations, with some regions slightly denser than others.

* Supernova shock wave: The most widely accepted theory is that a nearby supernova explosion sent a shock wave through the interstellar medium. This shock wave compressed the existing cloud of gas and dust, triggering the collapse.

2. The Collapse Begins:

* Gravity wins out: The small density variations within the nebula created slightly stronger gravitational pulls in those regions. This led to a slight inward pull, drawing more material towards those areas.

* Increased density, increased gravity: As more material was drawn in, the density in those regions increased. This increased gravitational pull even further, feeding a self-reinforcing cycle.

* Rotation and angular momentum: The nebula also had some initial rotation. As it collapsed, the rotation sped up due to the conservation of angular momentum. This caused the nebula to flatten into a disk.

3. The Formation of the Sun and Planets:

* Central core: The dense core of the collapsing nebula eventually became so hot and dense that nuclear fusion ignited, marking the birth of the Sun.

* Disk formation: The remaining material in the disk continued to clump together due to gravity. These clumps became the building blocks of the planets.

* Planetary growth: The growing planets continued to accrete material from the disk, sweeping up dust and gas until they reached their current size.

In summary, the solar nebula's collapse wasn't due to a sudden imbalance of forces, but rather a delicate balance that was tipped by the following factors:

* Density variations in the nebula: These created slight imbalances in gravity, causing the initial collapse.

* Supernova shock wave: This compressed the nebula, providing the initial push towards collapse.

* Rotation and angular momentum: The nebula's rotation played a crucial role in shaping the disk and influencing planetary formation.

The collapse of the solar nebula was a complex process, driven by the interplay of gravity, pressure, and initial conditions. It's a beautiful example of how seemingly small variations can trigger massive events in the universe.