You showed up too early - while some might appreciate the eagerness, I do not. Corporate cuts have eliminated the receptionist in most small to medium offices. This means that the person you are interviewing with is the one that will be called to greet you at the door and get you settled in. A good executive prioritizes their tasks and runs a tight schedule. If the prospective employee shows up 15 or 20 minutes early, they may put unnecessary pressure on the person they are interviewing with. Small and growing offices may have expanded into the "lobby" and "the conference room" and there is no true place for the interviewee to sit and wait. It's a good idea to arrive early but stay in your car until just a few minutes before you are supposed to meet with the hiring authority.
You peed on the seat - I swear this happened. The candidate was not hired.
You did not follow directions to the facility and then called the interviewer to walk you through them because you were late and lost - I really don't even know why I sat down with this person when they finally made it to the office. Our office was a bit difficult to find on a GPS, so I explained that in an email to the candidate and told them to follow these simple directions, they did not. The candidate was not hired.
You spoke negatively about your old job/boss/co-workers - an interview is not a complaint session about your current or former situation - blaming or deflecting blame is a big warning sign.
You kept trying to fill in your ideas about the job you were interviewing for instead of listening to the need and requirements of the position - just because the title is same, it does not mean that the duties and expectations from one company to the next are. Eagerness to demonstrate your qualifications is one thing but constantly going into specifics that don't apply the current position is a big no no.
There are dozens of other reasons but I suspect they have been covered ad nauseam . These are a few interesting tid bits to help along the journey.