Somewhat odd was that it was created by an amendment to the Colorado Constitution, and was voted on at a statewide referendum in November 1998, so technically was imposed by the voters of the State without those in Broomfield having a say so.
Technically, that is? Did Broomfield residents vote in favor? If not, that would be pretty crazy indeed, a county created against the stated preferences of its inhabitants.
I thought that it had been approved through the initiative process, with citizens/officials of Broomfield promoting it. But the Blue Book said that when the city had come up with the idea, that they had considered using the initiative. But the measure was actually referred to the voters of the state by the legislature.
This might have simply been proactive, so that provisions could be added that would protect the interests of the surrounding counties. When a city annexes an area, the area remains part of its county. When a city and county annexes an area, it is removed from its previous county. In the case of the city and county of Denver, this caused a lot of conflict (in the case of Denver, its school district was also extended). The Broomfield measure provides a way for the surrounding counties to be involved if Broomfield wants to annex some more areas.
I looked for some county vote totals, but all I could find was the statewide total (roughly 700,000 to 400,000). If Broomfield was actively against the idea, I doubt that the legislature would have proposed the measure, or that the voters would have approved it.