There has been a long standing issue between Google / Chrome (and now also Mozilla / Firefox) and Symantec regarding the SSL certificates that Symantec have issued in the past via their various subsidiaries (eg. Thawte, RapidSSL). In a nutshell, as of v66 of Chrome, certain SSL certificates will become distrusted (those issued prior to June 2016), and the browser will tell you accordingly. Similar will occur with Firefox v60 (or so I believe). Other certificates will get similar treatment later in 2018.