Earlier this week, Bitcoin experienced its largest upward difficulty adjustment since January 20th of this year. This large difficulty adjustment can be attributed to two key factors:
Miners that capitulated their rigs this summer, sold them to buyers that were better capitalized and likely had access to cheaper electricity. Weak miners sell more Bitcoin than strong miners, and a fair amount of ASICs shifted hands during the capitulation this summer.
New ASICs (XPs) are likely beginning to get shipped and plugged in.