The phrase to crawl, or to come, out of the woodwork means, of an unpleasant or unwelcome person or thing, to come out of hiding, to emerge from obscurity. The image is of vermin or insects crawling out of crevices or other hidden places in a building.
The earliest instance that I have found is from one of the unconnected paragraphs making up the column New York Day by Day, by Oscar Odd ‘O. O.’ McIntyre (1884-1938), published in The Index-Journal (Greenwood, South Carolina) of Monday 1st December 1930:
They were discussing one of those omniscient Broadway party-crashers—the sort that seems to crawl out of the woodwork everywhere. “The only way to keep him out,” said Bert Lahr, “is to use plenty of putty.”
Persons of unsound mind do sometimes believe that people or things come out of the woodwork. For example, on Saturday 3rd June 1843, The Bristol Mercury (Bristol, England) reported that “a well-dressed and gentlemanly man, who gave his name Isaac Boys, was charged with wandering abroad, he being of unsound mind”:
The defendant […] said that the sergeant at the station-house, when he asked him for water, gave him some stuff like alum, which burnt his breast dreadfully. A lot of men also kept popping in and out of the wood-work to try his nerves […]. Mr Brady, surgeon to the hospital, was sent for to examine him, and in a short time the unfortunate gentleman became so outrageously mad that it required several constables to hold him down. He was removed to the lunatic asylum.
Charles McGuirk also evoked this in a short story titled Hero’s Wife, published in the Oakland Tribune (Oakland, California) of Saturday 27th March 1937; the hero, Harry, “was a little bit of a guy. He stood about five feet two and he weighed about a hundred”; he once had an argument with a “big truck-driver”:
The truck-driver laughed and Harry hauled off and popped him on the nose.
The blow didn’t hurt the driver, but he was a hard drinker and often when he was on the verge of the delirium tremens, he saw little men like Harry coming out of the woodwork. And he thought to himself, “My God, if these dizzy things are gettin’ so they can sock me, I must be goin’ nuts.”