With all due respect, I just Googled define "due cause" and went to the first dictionary definition, where it says...
due adj. 4: Meeting special requirements; sufficient: "We have due cause to honor them".
If that's not clear, the preceding definition #3 says In accord with right, convention, or courtesy; appropriate. And in the same dictionary you'll find cause defined as grounds for action; motive; justification.
So you could paraphrase due cause as appropriate justification. Note that it's often used in legal contexts, so "appropriate" in this expression usually means right and proper, according to law.