I am sure you have since figured this out, since it’s been several years since you asked, but I had this question earlier, and more research answered my question, so I figured for others who may have a similar question, I’ll answer with what I found.
So apparently, some of Earth’s oldest meteorites contain xenon-129, which is a gas, even at relatively low temperatures, and it binds to nothing since it’s noble. This means that it wouldn’t have condensed and mixed with the rock during accretion, and therefore we must conclude that it is a by-product of radioactive decay. The parent isotope is iodine-129 which has a relatively short half-life relative to the age of our solar system (17 million years). Furthermore, it’s a very heavy isotope, so it’s likely from a supernova explosion. Since there was a short time between the explosion and the formation of the solar system, it is believed that such an explosion sent shockwaves through our nebula, therefore triggering the rotation. From there, gravity took over to condense the nebula, and the rest is just the Nebular Theory.
In general, it appears to me that things tend to trigger the events, but it may very well be in part due to thermal fluctuations, though the average temperature of the universe is about 3K, which is barely above absolute zero, and therefore might be negligible because of its low density, though I wouldn’t quote me on that.
Source: The Cosmic Perspective, Bennett, et al.