Those old needles are thin and light.. i'm not sure what they were made out of.. bakelite perhaps?
The most common cause of the breaking would be a combination of two things:
#1: Material fatiuge (the needle drying out and getting brittle)
#2: WSS (Wobbly speedo syndrome) thats where the speedometer needle "bounces" instead of operating smoothly.. usually caused by a dying speedometer, a failing speedo cable, or a jacked up speedo gear in the tranny.
So imagine an old, brittle twig in your hand.. if you gingerly move it around, it'll stay in one piece.. if you shake the [censored] out of it, it'll probably break.
Before you replace the needle, i'l check to make sure you have a smoothly operating speedometer first.. because if you have to buy a rebuilt speedometer or a NOS one, it'll come with a new needle, anyway.
