If you're referring to when Obito says "You're always too late when it's important" it's never revealed what Minato was doing then. Obito asked that because he expected Minato to help save Rin.
if you're referring to the flashback panel, I think Obito is expecting Minato to save Rin back then..
that's why Minato is breaking a sweat 'coz he feel guilty for his shortcomings..