Detect if you're being debugged on OS X. Apple's AmIBeingDebugged() won't handle a debugger not using ptrace and is insufficient.

osx_being_debugged.c

#include <mach/task.h>
#include <mach/mach_init.h>
#include <stdbool.h>

static bool amIAnInferior(void)
{
	mach_msg_type_number_t count = 0;
	exception_mask_t masks[EXC_TYPES_COUNT];
	mach_port_t ports[EXC_TYPES_COUNT];
	exception_behavior_t behaviors[EXC_TYPES_COUNT];
	thread_state_flavor_t flavors[EXC_TYPES_COUNT];

	exception_mask_t mask = EXC_MASK_ALL & ~(EXC_MASK_RESOURCE | EXC_MASK_GUARD);
	kern_return_t result = task_get_exception_ports(mach_task_self(), mask, masks, &count, ports, behaviors, flavors);
	if (result == KERN_SUCCESS)
	{
		for (mach_msg_type_number_t portIndex = 0; portIndex < count; portIndex++)
		{
			if (MACH_PORT_VALID(ports[portIndex]))
			{
				return true;
			}
		}
	}
	return false;
}

I haven't tried this code in a long time. I assume it's crashing in task_get_exception_ports() or mach_task_self(). Maybe you're calling it too early in the process since you're being DYLD_INSERT'd but that's a guess.