You could also have a year where you sell a house in excess of the gain exclusion, sell a rental property, cash in some stock options, convert an IRA into a Roth, sell a bunch of appreciated stock, sell a small business, or sell some other capital asset.

Those "one time" activities will all temporarily boost your AGI, meaning people come into and out of the "top 5%" brackets pretty regularly. It won't get you into the top 0.001%, but could easily boost what you think of as a "normal" two-income couple into the top 5% or 1% at some point in their life.