Spotless Chart, page-206

You're pretty much spot on. Basically a worse credit rating means rates go up. If rates go up then investors are obviously more inclined to pull out of risky shares yielding 5% and put their money back into savings. I haven't researched Spotless debt arrangements in detail but if they have a variable loan then that will eat into their profit. I currently hold this stock and beleive rates will go up in the next few months but not enough to justify pulling out of a 6% yielding stock.