They apparated into a field but the hunters weren't already there waiting for them - your memory seems to be mistaking you. They actually set up camp and were talking for quite some time. It wasn't until Harry said "Voldemort" that the bad guys showed up.
"Come out of there with your hands up!" came a rasping voice through
the darkness. "We know you're in there! You've got half a dozen wands
pointing at you and we don't care who we curse!"
So after Harry said Voldemort (the name being Taboo at this point) the 'hunters' show up and have the place surrounded. Hermione has time to perform a spell to make Harry not so recognizable but there probably wasn't time for them to form a quick plan and apparate away from there in a group. Ron and Harry didn't have their wands ready either so they really couldn't do much at the time.