Why do TVs flash when they turn off?

On older sets that use a CRT (cathode ray tube) the magnetic field collapses while the ray gun is still "firing" (trying to display images). This poses no harm to the set nor the user, just part of having an older set.

It is also not a manufacturing defect in any way... it's just what older CRT sets do when they turn off.

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This occurs when the power is removed from the "aiming" control, which directs the beam of energy to each individual phosphor cell (i.e. pixel) on the screen, these being arranged in lines across the screen. The magnetic field drops before the CRT's "electron gun" has fully discharged, allowing a brief stream of energy to reach only the cells across the middle of the screen, which typically become illuminated in a white flash.