Sporadic contrails. Ambient Air temperatures, wind velocity, barometric pressures, and several other factors contribute to this phenomenon. It's why they're spread out like morse.
Used to see this a lot when I lived in San Diego. I was only 10 miles from MCAS Miramar. Huge Air Base. They flew training sorties daily, C-17's, Hornets, Blackhawks escorted by 4 Apaches, Ospreys, etc. This kind of trailing was a very common sight.