Once you've played Ranked and finished your provisionals, you now have a set MMR. When you come into this season, your placement will be based on your MMR, so if you already had done Ranked before it'll be based off that. If this is your first season you'll be given a baseline MMR which is somewhere around mid or low Silver.
Since you have played Ranked before (at least since S3) you already had MMR, but it was lower than the baseline that a new player will even get.
To avoid being placed in Bronze for a new season, you need at least Silver MMR.
I doubt I explained that as well as possible, but I'll answer any questions you have and can provide the support article links.
With the new season, everyone loses some MMR for the soft reset. The picture below is Lolking's graph of your estimated MMR. You can see that you stopped playing Ranked before last season ended, then on Jan. 7th the soft reset happened and your MMR went down. On April 14th you finished your placement matches and were put in Bronze since your MMR was way down there, then we see you quickly climbed back up to Silver.
https://i.gyazo.com/fa548651a1b24318cf88672005d871a7.png
Your MMR right now is even lower than it was last time. Unless you raise it before the next season starts you are going to get placed in Bronze again because of the MMR soft reset.