Well, the pipeline MLPs are being downgraded because they have run up in price. Since they are primarily valued on the expectation of dividend payout, there isn't much price appreciation left in the short run.

In addition, because of the runup in price, they are selling new "units" or shares, which dilutes their earnings. This is typical of MLPs, though. Since they pay out their profits, they have no source of funds other than borrowing or selling new shares. They tend to sell share when the market is good, not when the market undervalues their shares. The presumption is that they'll do something with the money that, in the long run, more than makes up for dilution of the shares.