Street Grimoire, pg. 202, states that whenever a spirit becomes uncontrolled, the GM rolls to see if it becomes a free spirit.
So, I got curious, how does a spirit become uncontrolled?
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The rule part you quote refers to "uncontrolled" (which is usually for vehicles without a pilot(software) but in the next sentence refers to bound and unbound spirits, which gives the implication that uncontrolled is to be read as "Spirit that does not owe services to someone at the moment".
To achieve this, there are several ways. The most relevant are to Dismiss a spirit or glitching on the summon or (re)binding roll.
DISMISS SPIRIT This is the action of freeing a spirit from the summoner’s control. It does not immediately send the spirit back to its home plane but instead frees it to do as it chooses.SR5 core 165
GLITCHES A glitch on conjuring can result in [...]. More punitive (read: evil) gamemasters may see this as an opportunity to introduce the magician to a spirit of the intended Force who is not under the summoner’s control and wishes to have a “conversation” about how some spirits feel the practices of summoning and binding is a form of slavery.SR5 core p300
Atop that, there is one more: Kill/KO the summonerSR5 Street Grimoire p184, as this gives a break free chance to void the services still owed. In a similar vein: if the caster loses his magic attribute at least fettered spirits lose their bond.
Once a Long-term Bining runs out, the spirit also ends uncontrolledSR5 Street Grimoire p192 and is thus able to become a free spirit.
Fettered and Ally Spirits also have the option to break free under some conditionsSR5 Street Grimoire p192/202.