Okay...well that was...expected. So what's worse? Obito becoming juubi jinchuuriki or madara reviving? I'm pretty sure madara has another few plans up his sleeve. Pretty bummed about obito though. Everyone saw it coming a few hundred chapters ago that he was the "masked man" against the fourth. How did the fourth know though He sounded surprised even after he used the jutsu...weird.
Yes but how would he know that his jutsu was on obito. Back then he fought the "masked man" and put the seal on him. But here, he teleported to obito. Aka he probably knew there was some sort of corrolation between the two. But he seemed so surprised when he got there for some reason...
Dude... He wanted to teleport to the Madara. Why? Cuz everyone else was also racing to prevent Madara's jutsu. Next, remember how he initially believed that Madara was behind the mask? So he teleports to the masked man who he had marked all those years ago. But lo and behold! He reaches Obito instead of Madara! Then he puts 2 and 2 together and realises that Obito was behind the mask all along... Bear in mind this is the first time Minato meets Obito after his resurrection via Edo Tensei, up till this point he had no notion of Obito being alive.
Minato realized that the 'masked man' was behind everything. He was able to teleport to him because he fought the 'masked man' all that time ago. He also understood that the 'masked man' was not actually Madara, but someone else.
His shock came in when he realized the 'masked man' was actually Obito, his student. Two different realizations in a short time span