Originally Posted by FBinNY

Two of the common cause of front brake endoscopy are not having enough weight on the saddle, and letting the bike stop but not the body.

The first is pretty obvious, if you don't have your ass firmly on the seat, it let's the rear wheel lift in response to the front brake force. Once that begins, the geometry changes, and less force is needed to continue the roll.

The second is vertical different. The bike weighs between a fifth and tenth what you do. So very little is needed to stop it. If you don't brace with your arms, the bike stops suddenly, but you keep moving forward, until you go as far as you can, then your momentum takes you and the bike into a rollover. It's analogous to a passenger in a car without a seat belt hitting the windshield, but plays out differently.

The second is pretty common, and a good reason to take descents in the drops. You get to descend faster, but are in a properly seated and braced position so you and the bike are one.