One of the worst performers on the ASX 200 on Tuesday has been the Westpac Banking Corp ( ASX: WBC ) share price. In morning trade the banking giant's shares are down almost 5.5% to $26.25. Why are Westpac's shares sinking lower? Although the market is a sea of red today and peers such as Australia and New Zealand Banking Group ( ASX: ANZ ) and Commonwealth Bank of Australia ( ASX: CBA ) have dropped lower, the majority of Westpac's decline can be attributed to something else. This morning the bank's shares went ex-dividend for its fully franked 94 cents per share final dividend. Eligible shareholders can now look forward to receiving this generous dividend in their nominated accounts on December 20. Federal Court news. In addition to this, news that that the Federal Court has refused to approve a $35 million penalty given to Westpac after it admitted to breaking responsible lending laws could be weighing on its shares.