A ninepin is one of the wooden pins used in ninepins, a bowling game played with nine pins arranged in a diamond pattern.
The game predates modern ten-pin bowling and was once common enough that "ninepins" became a generic term for bowling.
The pins fell like ninepins means things collapsed easily or in rapid succession.
1. bowling — skittle, Pin for bowling; alley (Narrow way between buildings); knockdown (Blow causing a fall); wooden (Made from timber); lane · ball · roll · throw · frame · score · spare · strike · gutter · split
2. target — aim, Purpose or direction; knockdown (Blow causing a fall); throw (Propel by arm motion); accuracy (Degree of correctness); topple · standing · arrangement · formation · scatter · precision · hit · setup · reset · clearing