Mr. Link Deas is a man of integrity as well as a Maycomb businessman who likes to keep his town peaceful and his business reputable. Therefore, when he hears the outrageous accusations against his employee Tom Robinson made by the reprobate Bob Ewell and his daughter, as well as the harsh interrogation of Tom by Mr. Gilmer, Mr. Deas feels compelled to speak out.
That Mr. Deas is concerned about the case against Tom Robinson has been previously evinced when he is among the men who come one Sunday to talk with Atticus. He is worried about the Old Sarum bunch and wonders if the trial could be moved to another location:
"...I'm worried about...can't you get a what is it, Heck?"
"Change of venue," said Mr. Tate, the sheriff.
Certainly, it is an impulsive act of Mr. Deas that he swiftly stands and speaks up for Tom in court. But, knowing that Tom is quiet, honest, and faithful, Mr. Deas is incensed when he hears the vicious accusations of Bob Ewell. Then, when Mr. Gilmer insinuates that Tom's having run off when Bob Ewell came home demonstrates his guilt, Mr. Deas stands and defends Tom:
"That boy's worked for me eight years an' I ain't had a speck o'trouble outa him. Not a speck." (Ch. 15)
Immediately Judge Taylor silences Mr. Deas and orders him from the courtroom lest a mistrial be declared.
Link Deas was Tom Robinson's employer, and he is one of the characters in the novel who most strongly represents tolerance and the shaking of the status quo. The way he represents this is by speaking out of order at court on behalf of Tom.
Mr. Link Deas rose from the audience and announced: 'I just want the whole lot of you to know one thing right now. That boy's worked for me eight years an' I ain't had a speck o'trouble outa him. Not a speck."
This caused a problem in court since he would have accidentally caused a mistrial for not following the proper protocols to speak as a witness. He also got a harsh reprimand by the judge.
Shut your mouth, sir!" [...] "If you have anything you want to say you can say it under oath and at the proper time, but until then you get out of this room, you hear me?"
Basically Link was fed up with the trial, wanted to see justice served, and he was not scared to speak his mind. Remember that, eventually, Link not only gave Helen Robinson a job--he also ran Ewell off his property and threatened him if he dared to ever bully Helen. Link Deas is, essentially, a truly good man.