Since Boruto (and Sasuke) respectfully know nothing about the seal: What it does, how it activates, if it does anything harmful, etc. There's probably a big chance that we might slowly learn these things as the series progresses, since this will probably be Boruto's main source of power for fighting Rikudou-beings, and overall just enemies who require more than what a Genin is capable of.
If it goes that way, then he might activate his seal under duress. For example, if a close friend or family were injured and he was filled with rage, I can see this being a catalyst. Or if Boruto is surrounded by enemies who are about to kill him. Another one would be him getting attacked by an Otsutsuki and Sasuke nor Naruto are there to save them.
However, Sarada & Mitsuki respectfully are probably going to learn about the seal once they witness him using it for the first time. Which will probably be to protect them from something, I suspect. Boruto will probably faint or something, and Naruto will get involved after Sasuke spills the beans.
Just guessing though.