There are a number of reasons a dog may hang out under the bed or other furniture:
Fear - many animals, not only dogs, feel safer in dark, close spaces when frightened. This behavior is usually a temporary response to a sudden, frightening change, such as a loud thunderstorm or fireworks on the 4th of July.
Illness - similar to fear, many animals seek quiet, private places when feeling unwell or in pain. This behavior may last a few hours or several days; it's definitely cause for concern if it's a sudden change with no other explanation.
Sulking - a dog who has been chastised -- either by his people or other animals in the household -- may retreat to sulk in private. Similarly he may be hiding out if he believes he is going to be chastised -- if, say, he knows you have just come in the front door and aren't likely to be too happy about how the stuffing from the couch is all over the living room floor... again.
Temperature - the spaces beneath furniture may provide a cooler spot to hang out in when temperatures are high; likewise a confined space may be easier to warm up with body heat when it's cold out.
Idiosyncrasies - some dogs just plain like to be hidden when relaxing, eating or sleeping, much like a wolf with a den. While some breeds, such as terriers or huskies, are more prone to such behavior than others, it is not unusual for any canine to do so.
In short it's difficult to say why any particular dog has taken to hanging out beneath furniture without knowing details about the circumstances.
Any sudden behavior change should be looked into more closely; watch for symptoms of illness, pain or distress, such as rapid breathing, excessive thirst, disorientation, lethargy, and reluctance to move. If in doubt, a trip to the vet won't hurt.
Fear responses are usually fairly obvious, but can be quite tricky in some circumstances, particularly when bringing home an adult dog with an uncertain history. The dog may be unreasonably afraid of things you wouldn't ordinarily suspect; children playing nearby, the presence of other dogs, the sound of a certain kind of car engine -- it can be challenging to identify the cause, as well as to help the dog overcome his or her fears.
If it's not a sudden change then it's most likely just personal preference and isn't cause for worry; unless, of course, the occupants on the surface of the bed find its doubling as a domestic wolf den undesirable. Adding a few blankets beneath any semi-enclosed space -- a desk, a small table, the space beneath the stairs -- will help encourage relocation.