Well, according to Mrs. Gardiner's letter to Elizabeth in ch.52, Wickham "scrupled not to lay all the ill consequences of Lydia's flight on her own folly alone". Now, of course I take that with a handful of salt ;-), but I do think her own foolishness had something to do with it. As Elizabeth reflected back in ch.46:

"Surprise was the least of (Elizabeth's) feelings on this development. While the contents of the first letter remained on her mind, she was all surprise -- all astonishment that Wickham should marry a girl whom it was impossible he could marry for money; and how Lydia could ever have attached him, had appeared incomprehensible. But now it was *all too natural*. For such an attachment as this she might have sufficient charms; and though she did not suppose Lydia to be deliberately engaging in an elopement, without the intention of marriage, she had no difficulty in believing that neither her virtue nor her understanding would preserve her from falling an easy prey.

(Elizabeth) had never perceived, while the regiment was in Hertfordshire, that Lydia had any partiality for him; but she was convinced that *Lydia had wanted only encouragement to attach herself to anybody*. Sometimes one officer, sometimes another, had been her favourite, as their attentions raised them in her opinion. Her affections had been continually fluctuating, but never without an object."