Campaigns end up in spam folders for several reasons, including the sending email address, subject line, or message content.
Even legitimate and innocent campaigns can get trapped in spam filters based on small issues you may not have considered, like your sending name or email address, and information shared from other spam filters. While we can't tell you specifically if your campaign went to junk folders, your Reports page contains a lot of helpful information that can help you improve your campaign deliverability.
If a lot of your recipients click the Report Spam button in their email client (e.g., Gmail, Yahoo), those services can start to block your campaigns and deliver straight to the spam folder.
If a recipient tells you he or she clicked Report Spam by accident, many email services include a Not Spam option. Ask the recipient to find the email in the spam folder, and mark it as Not Spam or Not Junk. Buttons and labels will vary by inbox. This can help keep future messages out of other recipients' spam folders, too.
If the recipient doesn't contact you directly, just call it a lesson learned. Although MailChimp can tell you who's complained about your campaigns, we don't recommend following up with those subscribers.