Dont Blame The Manager

Managers are responsible for getting their projects done on time and on budget, and so they are given authority over many aspects of the project. The manager's superiors and subordinates hold the manager accountable for decisions made. However, lots of things happen that are outside the manager's control:

Project Budget and Schedule -- These are often set by higher-ups, or were set by a manager who previously held the position. They may be impossible to change, due to contractual obligations or financial realities.
Personnel -- In many organizations, the manager is assigned people to work on the project, or is only allowed to select from a small pool of available people (who are often available only because no other managers want them). Many managers are not allowed to hire new people, and there are legal and procedural barriers to firing people. Key people may leave or become unavailable unexpectedly. And there is little a manager can do to make people more productive or to get them to work together effectively if the people don't want to do so.
Customer Whims and Changing Marketplace -- Customers change their minds, and the manager has to deal with that by either changing their minds back, or by changing the scope or timetable for the project.
Company Policies -- Unless the manager is a high-level executive, the manager has no control over formation of the policies and procedures enforced by a company, but is responsible for ensuring they are followed.
Company Politics -- Most managers have no part in the Machiavellian schemes that go on within the company. They're too busy doing their jobs. Unless there is strong evidence to the contrary, assume that your manager is not in on any of the sinister plots that adversely affect you, your project, or people you care about.
Product Quality -- A manager can set high standards, but can't force team members to meet them. And if budget and schedule are inadequate, quality inevitably suffers.
Random Events -- Unforeseeable natural and man-made disasters and accidents happen. Good managers will have contingency plans in place to mitigate the effects of random events, but can't plan for everything. Managers are expected to deal with whatever consequences arise, which often requires changes to the project plan.
Experience -- Managers are sometimes thrust into positions for which they are unprepared. They do the best they can, but trial-and-error learning often generates a lot of error. A manager in this position may be considered "incompetent", but the real blame belongs to the higher-level managers who put inexperienced managers in such positions without adequate training and support. See InexperiencedManagersAreRampant.
Multiple "Interested" Parties -- A manager often has many people to keep happy. Often times new ones just pop up and make brand new demands. If a manager seems to change direction, it may be that he is trying to find a middle of the road approach that will, if not keep everyone happy, at least not make any party too angry.