The Imperial fleet dropped out of hyperspace right after Jorin Sol's attempted assassination of the commanders on board Rebel One. It began firing on the Rebel fleet, destroying transports and smaller capital ships en masse.
A regretful Jorin Sol managed to convince Tungo Li to order all Rebel ships to scatter into hyperspace rather than follow the now-revealed escape algorithms, which would only bring them to another Imperial fleet.
Contrary to Vader's expectations, starfighters under Wedge Antilles were able to hold off the Imperial fleet long enough for Sol to overcome his conditioning and program the damaged Rebel One to jump into hyperspace. Luke Skywalker managed to convince Sunber to let him and Princess Leia Organa go, and the Rebel ships were able to escape through hyperspace and scatter throughout the galaxy. Much of the fleet was deemed lost, possibly for good.