Historical Rules · 1 February 2013
Rule 603.2d
Official source text from the 1 February 2013 Rules release
Some trigger events use the word "becomes" (for example, "becomes attached" or "becomes blocked"). These trigger only at the time the named event happens -- they don't trigger if that state already exists or retrigger if it persists. An ability that triggers when a permanent "becomes tapped" or "becomes untapped" doesn't trigger if the permanent enters the battlefield in that state. Example: An ability that triggers when a permanent "becomes tapped" triggers only when the status of a permanent that's already on the battlefield changes from untapped to tapped.