There are a few reason why the stock price may decrease after a divident is paid:
- Some people holding the stock may have been waiting to get the dividend before they sold the stock.
- Stocks that pay dividends are making the choice to reward the stockholder with cash instead of reinvesting profits into the company to grow its business. When a pile of money gets paid to stockholders, that's a signal that the long term growth potential is decreased, at least a little bit.