I only have anecdata, but I have identified two factors: isolation and vulnerability, or rather the sudden change in how isolated or vulnerable we feel. We become accustomed to the sounds of our own neighborhood and city in general, so if you stop all these sounds and introduce new ones, in addition to literally being isolated and unable to read others’ emotions on the situation, our brains generate discomfort via anxiety and stress. The vulnerability aspect is due to the fact that we don’t know what’s out there. When we’re walking around in the darkness, our brains are determining that we are in danger and attempts to signal to us that we need to GTFO. Being in terrifying situations will heighten that response.