Primary Cause ? Not one, but suspect it's business related.

Much like a small business suffering from having gone too big, too fast. Where critical infrastructure was not in place to carry out Gus's plan. From wrong selections being made concerning coaches, to wrong selection of players, etc. For instance take offense, he began with a viable plan; yet a failed defense created an ever increasing vacuum for his time. There was also the failure to establish a stable work environment as management became an ever revolving door. Part of this was inherited like the flow of DC's from Roof, Van Gorder, Johnson, Muschamp, to Steele. However throw in nepotism and you have a business at risk. Especially when the chosen protegee fails to not only produce, but is also deemed immune to failure. It was rumored such favoritism hampered cohesion within management, as a hole developed from someone not doing their job. If memory serves me correct, smart managers are those who's business will run unaffected without them having to be there. Hard to say such independence was ever developed (QB's for instance).

Until now we're to the point where future strategic development is likely to suffer... including lack of capital and increased competition for limited raw resources, including future draft choices likely being withdrawn. Such an inability to address short comings has customarily led to loss of market share and eventual collapse of the business. Just a guess but I imagine time is right for a hostile takeover.

So I'm guessing Gus really never stopped being OC/QB coach long enough to become (an HC) responsible enough to run the entire company... at least that's my guess for single point of failure.