Different services run on different frequencies. Some services using modems need return communication using another set of frequencies.
It's possible that certain problems or damage only affect certain frequencies.
For example, small breaks in the cable or loose connectors will affect low frequencies before they affect higher frequencies. Return or upstream communications use the lower 5-45 Mhz frequency space.
On the other hand, higher frequencies are effected more by corrosion and other things that damage the copper skin of the center conductor of the coax cable. So when the cable equipment gets wet internally, higher frequencies 550 Mhz and up which are used by services such as digital SD and HD channels go out first.
In most of the powered cable equipment (nodes, amps, and line extenders) used for signal distribution in your local neighborhood, the forward (50Mhz and up) and return (5-45Mhz) signals are split and follow different paths within the equipment. The techs can then adjust them separately.
Beyond that, all the different channels are combined in several different spots at the local cable headend and further upstream from there. Depending on where problems happen it can knock out specific services, groups of channels, individual channels, or just parts of channels such as audio, video, closed captioning, etc.
The Cable 101 links listed here should help:
If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes.