Hi Afi,
I can say with confidence that what you're experiencing is actually peeing, not ejaculation. For one thing, ejaculation usually feels VERY different from just going to the bathroom. And for another, it's not going to happen without some kind of stimulation. You can read more about it here:
Squirt: On Ejaculation.
What may be happening is that, when you feel arousal, you become more aware of what's going on in your genitals and they become more sensitive. Those things, plus the increase in blood flow to the area, may make it more likely that you feel like you need to go to the bathroom.