Boeing released a highlights real of its test flight, and around 2:30 the main chutes deploy. When that happens, it appears as though is lightning like flashes around the main chute's cords. What is causing that? Video is here
Those "lightning flashes" around the main parachute's cords are actually the cords being illuminated by its blinking strobe lights. Now I can't find a document that specifically says Starliner has strobe lights but there are pictures of Starliner where its parachute cords seem to be illuminated.
In this video, you can see the strobe lights blinking:
There is also a cord that is violently swaying (it is moving at hundreds of km/h in an atmosphere) and when it gets illuminated by the strobe light, it looks like a lightning bolt.
The "lightning bolt" that is circled is actually a cord that is swaying and is being illuminated by a strobe light.