Q. Since the announcement was made after the close of the market, you need to look at the following day's prices. The average investor expected a higher dividend announcement. In this case, the announced cash flows were lower than expected. Thus, the price, the sum of discounted expected cash flows, declines. The average investor believed that an increase in dividends was not a good decision.
Investors react negatively to bad news by selling the stock.
Other things remaining equal, this would lower the market price. A regular dividend becomes part of investors' expected future cash flows, in contrast to a one-time dividend distribution. You might want to consult your favorite textbook for additional arguments and models of dividend policy.