The one guy ran in yelling at Mon Mothma and then they almost all send their fighter support. What exactly happened?
The person who ran in told her that the Rebels had made it to Scarif:
“Intercepted Imperial transmission, ma’am,” the private answered. “Rebels on Scarif.”
Scarif? How was that possible?
But the answer was obvious. She saw it on Draven’s face, too, and Merrick’s. While Mon had spent the night clutching like a miser at whatever pieces of the Alliance she might preserve, Jyn Erso had gone to risk everything she had.
Rogue One: A Star Wars Story
First, the fact that such a small Rebel team had made it as far as the surface of Scarif might have motivated others to see the mission as less hopeless. But more than that, it seems that many people (such as Raddus) had already gone:
She fixed the private with a sober look. “I need to speak to Admiral Raddus,” she said.
“He’s left already.” The man was almost stammering. “He’s in orbit aboard the Profundity. He’s gone to fight.”
“I see,” she said, and slowly smiled. Merrick’s expression was expectant; Draven’s grave and resolved.
Perhaps she had given up hope too swiftly.
Less than ten minutes later, sirens were announcing the departure of Red, Blue, Green, and Gold Squadrons along with the U-wing transports. Raddus had already contacted all capital ships within range of Yavin or Scarif. Draven had brusquely warned Mon not to think of joining the mission, no matter how inspirational she thought she might be; but the warning hadn’t been necessary. Mon understood her limits too well to get in the way.
Instead she reminded herself of her pride in the soldiers of the Alliance and watched pilots and infantry personnel and technicians scramble to their vessels. Anyone capable of contributing would find his or her abilities welcome in the coming battle.
Rogue One: A Star Wars Story
Thus Mothma was only bowing to the decision that many of the military commanders of the Alliance had already made.
Mon Mothma and her close advisers were extremely keen to attack Scarif. The Rebel Council (comprising senators and fleet commanders from a dozen different sectors) were less keen. They took a vote and Mothma ultimately lost.
When she discovered that Jyn Erso was already on the planet (having "gone rogue") she asked to speak to Raddus only to discover that he'd also left for Scarif, along with the pride of the Rebel Fleet, the Profundity.
Mon Mothma: I need to Speak to Admiral Raddus
Private: He's returned to his ship. He's going to fight!
Since the plan was already underway, and with the Council absent, Mon Mothma decided to accept the fait accompli and order the ships that remained in orbit to support the insertion team.
Draven and Merrick glanced at Mon Mothma then, and she had to do her best to suppress a smile. She was pleased that Jyn and Cassian had gone to Scarif to complete the mission the high command didn’t have the stomach to back. Now Raddus might also make sure they got the help they deserved.
The two generals scrambled off to see what they could do about it, but Mothma knew they were already too late. There would still be ships hurrying to catch up with Raddus in orbit, but none of them would refuse the admiral’s orders to join him in jumping to Scarif.