In the Miller’s Prologue, why is he having trouble keeping in the saddle as the pilgrims ride ?

The miller is struggling to stay in his saddle as he rides alongside the company of pilgrims because, as the narrator says, he is pale with drunkenness, which is making it difficult for him to stay in his saddle. If he is so drunk that it is making him pale, this suggests that he is drunk enough to feel sick and unwell—the result being that he "hardly sat" upon his horse. However, this does not stop him from saying that he is sure he will be able to tell a tale good enough to equal the knight's tale, which has just been told.
The host, seeing that the miller is extremely drunk, suggests that somebody else should tell a tale first, but the miller will not have this. He says that he will either speak now or leave the company. At this point, the host relents and allows the drunken miller to speak.