A few possibilities. And, as noted already, if this is a tiny percentage that's not impacting your business you could ignore it.

1. They're spam sign-ups. Have you noticed a wildly unrealistic uptick in overall traffic? You could be getting throttled by bots.

2. There's a problem with your email sender score, so your emails aren't getting through spam filters. What's your deliverability percentage?

3. There's a problem with the email itself:
-> subject line issues decrease open rates or can decrease clicks if there's disconnect between open and what the recipient actually finds inside
-> copy or design issues can cause clickthrough issues

4. There's a disconnect between what people expect when they sign up and what they have to do when they get that email. Do they know they're going to get the email? Is it worth the time for them to take that next step, depending on what value they expect to get from the sign up?