To be fair to Nolan: his showing up wasn't "random" per se, and he was actually a former test pilot of that very spaceship.
Yes, but IIRC 10 years had passed since he'd flown the thing, and he knew nothing about the mission, which was supposedly humanity's last hope. It's presented as if he finds out about it, gets a quick tour, casually says goodbye to his family and then takes off the next day. Maybe the original script explained this better, but as it stands it came across as poor writing.