He didn't jump back because he was physically incapable of doing so. In Eve: Online, other ships can use ship modules on you that prevents you from leaving the area. It's called "tackling." The pilot in question, upon erroneously jumping into the system, was tackled by enemy forces before he could escape. Instead of eating the loss, he called up on his allies to jump in to attempt to destroy the ships that were tackling him. (A titan-class vessel is largely unable to destroy the much smaller Heavy Interdictor-class vessels that are capable of tackling it due to the ship's poor tracking and large guns, and requires help if it is tackled. A good analogy here is trying to kill a fly with a cannonball at 30 kilometers -- guns in Eve work similarly.)
When the pilot's allies arrived, the enemies called THEIR allies and joined the fight. With the amount of tackling ships on the field, neither party could easily escape, and things snowballed considerably. The enemy forces in this case had the upper hand of available pilots and were able to inflict heavy losses.