Even with the hungry caterpillars, during a particularly nasty encounter with an asteroid storm, things can get through, I'm assuming. There's also sudden solar flares that can overwhelm even the strongest safety precautions, being inside a planetary system, one could get hit with those. Sadly, in space, lots of things could go wrong.

Amd why the two ships were there? They both could have been surveying the planet. Or they both could have been capturing asteroids for the resources the big rocks could provide for their peoples. Or they could just be on similar trade routes and just both ended up in trouble, who knows?