I would argue that Wilbur is not a strong-minded pig. From the very beginning of the story, Wilbur depends on other people and animals to keep him alive, to help him find his way, and to accomplish the things that he thinks are important. Being the runt of the litter, he is going to be killed but then Fern saves him through no real effort of his own.
Once he is grown, he is sent off to the Zuckerman farm and is only saved with the other animal's help and then Charlotte's help in particular. Even at the fair, it is Charlotte and Templeton who do the heavy lifting to keep Wilbur safe by helping him win the fair.
It is only then that Wilbur takes charge when he forces Templeton to put the egg sac somewhere safe where he knows that it will get back to the farm and ensure that Charlotte's children will survive.
Throughout the story, Wilbur's major concern seems to be staying alive and this happens mainly because of Charlotte and the others as opposed to any sort of planning or action on Wilbur's part.