Why was Wickham and Elizabeth's first conversation concerning Mr. Darcy ill-judged?

Elizabeth is naturally curious as to why Wickham and Darcy reacted so strangely when they met one another on the street, and hopes that her conversation with Wickham will provide some clues. Wickhams very cautiously begins to ask Elizabeth about Darcy, and after hearing how much she dislikes him, he decides little by little to tell the story of his previous relationship with Darcy and Darcy's father. All the while, he pretends to be avoiding the subject and pretends that, out of respect for Darcy's father, he does not want to say anything negative about Darcy or to publicize what has happened. Yet the fact that Wickham so quickly divulges all the details of the story to Elizabeth after having just met her gives reason to doubt the sincerity of his supposed reluctance to defame Darcy's character. Elizabeth, however, sees no inconsistency in Wickham's behavior, and readily believes everything that he tells her, having judged him to be extremely amiable and trustworthy.