Why do risky companies tend to have lower target payout ratios and more gradual adjustment rates?

Because their promises aren't reliable. For example a risky company with a high payout may not have enough earnings to make the promised payout. Some companies have payout ratios above 100%! You know that payout is going to drop, you just don't know when. In my opinion you should always look for payout ratios below 50%. Unless it is a REIT or utility.