Networks were able to call the Iowa caucuses early because they watched the early results from suburban precincts in Polk, Dallas and Story counties where Trump lost in 2016.
Some precincts allotted a few minutes to each candidate and quickly finished. Others allowed lengthy preaching and the preachers thought it unfair when significant numbers in efficient precincts had already been counted.
The projected it before most of the voters had voted. People actually left because they got the alert on their phones.