It's rather confusing actually. To do something "low-key" essentially means to do it in a tempered way. You can also use it with the same meaning as an adjective. For example: "She has low-key hated me for years." This means that "she" has kept her hatred restrained and in check, perhaps even tried to keep it secret and unsaid. "Crashed" most likely means that the person either fell asleep or otherwise just stopped maintaining any kind of mental or physical effort. He let all such efforts fall. To "low-key crash", then, doesn't completely make sense to me, but I could hazard a guess. It could mean almost, as in "I almost crashed" or perhaps better it could mean "I crashed in a way that was not complete and hard but rather more subtle and restrained." "Low-key" tempers and reduces what it modifies. Therefore, I find it not completely clear since I don't know what it means to temper or reduce crashing, but it might be like I suggested.