It is possible that margin requirements inhibit very small investors whose access to other forms of credit is limited. If so, the only effect of increasing margin requirements is to price out of the market the very small investor without addressing the broader issue of stock price bubbles.
If a change in margin requirements were taken by investors as a signal that the central bank would soon tighten monetary policy enough to burst a bubble, then there might be the appearance of a causal effect. But it is the prospect of monetary policy action, not the margin increase, that should be viewed as the trigger.