Why did this swarm of dragonflies form in this location?

Swarm of dragonflies over a field.

Phenomenon Description: A large swarm of dragonflies (a mix of common green darners and wandering gliders) flying over a grassland at an environmental education center in Raleigh.
Location:  Wake County, Prairie Ridge Ecostation 
Grade Level or Course: Grade 5  
Question:  Why did this swarm of dragonflies form in this location? 
Other related Questions:
Why does this behavior matter to our environment?
Do swarms look the same regardless of the species that make them up?
Crosscutting Concepts:  Cause and Effect: Mechanism & Explanation
Science Domain: Life Science 
NC Standard: LS.5.2 - Understand the interdependence of plants and animals within their ecosystem
NC Objective: LS.5.2.3 - Use models to infer the effects that may results from the interconnected relationships of plants and animals to their ecosystem.

Additional Resources: 
https://christinegoforth.com/portfolio/entomology/ (Includes YouTube Video of Dragonfly Swarm)
https://pages.scistarter.org/dragonfly-swarm/
https://www.wbur.org/hereandnow/2019/09/17/dragonflies-swarm
Image by Chris Goforth

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