Root bound caused root rot?

Iv been having alot of problems lately with nute deficiency and just weird behaviors with my plants getting brown spots, leaves turning over, leaves burning in a half, new growth falling off. I suspected root rot after going through all the possible things and nothing helping. I took a plant out and found a brown spot on the bottom of my plants roots but the roots are so thick. Is my plant root bound that caused the rot?
That was the worse one!