Question about betta flaring?

I was under the impression this was something they did in stressful situations.. I had a tank with a divider and a betta on each side.. the red one was flaring non stop and stalking the other betta which is blue. I gave them an adjustment period and 3 weeks later it's clear the red is dominant and the blue couldn't be bothered. So this weekend we separated them.. 2 different tanks, in fact 2 different rooms of the house. The blue who has never flared before is flaring non stop yet also has his 1st bubble nest going. He is giving me mixed messages...(typical male) someone help..