Some consonants such as *n,d,t* are usually alveolar in English.  
Except that they are replaced by dentals when they are before dental fricatives(th).   
Examples: *tenth, said this, in the...*  

What about "r" before "th"?   
Examples: *Arthur, north, northern.*  
My conjectures are:  
"a" or "o" are lowered in the mouth so that we have ɑ or ɔ .    
the mouth becomes less tensed  
the "r" is very loose  
and "n" in "north" becomes dental!