Do you have any evidence for either of these answers? They don't match with the reasoning for market circuit breakers I've read in the past. I also don't understand why a collapse in bond markets would cause an equity markets spike that could trigger an upside circuit breaker. Or, for that matter, why a down bond market would cause an equity market crash.
Also, when people have short positions in the equity markets they get margin calls and are forced to close out their positions by buying stock when the market rises. This would be a similar process that roymurdock mentioned but to the upside. It's called a "short squeeze."
The underlying logic is that capital would shift from bond to equity markets, and while that's perhaps a little oversimplified I think the point is still valid. A bond market crash would trigger a risk off move [1] in which safe haven assets such as gold, the dollar, and in fact government bonds would outperform while risk assets such as equities are generally sold.
