The HD "generator" is a computer, some more reliable than others. Most run a version of Linux, customized for Ibiquity's needs. Being a computer-They occasionally need a kick start. Early models have standard IDE disk drives, which can be filled up, or become corrupt etc.
The transmitters, are usually capable of IBOC only, IBOC +FM or Analog FM only. Some transmitter brands are seemingly more reliable than other brands when producing IBOC. Occasionally you find that you do, literally, have to reboot the transmitter, if it gets into some sort of funk, Starving the IBOC of data is usually source of "funk". I have seen multiple transmitters running iboc that produce fire and smoke, or they return to "packing crate" status...which means that the transmitter is no longer on your channel, but some other channel. Fortunately most of them will not run on the wrong channel.
On top of all of that the HD-2 HD3 etc are usually another computer. Which is another chance for a lockup. THEN, on top of that you probably have an Automation playback machine...another PC, that will need a kick now and again.
Between the analog, streams, video streams, and HD these side channels are easily overlooked.