I had this problem,; it started with 15.0 for me, iPad mini 4. I found that the new SHELF for apps showing up at the bottom of the page was my culprit.
I had tried clearing the cache, history, un-synching, all of those things others have mentioned here.
My solution was to double-click the home button and bring up all the tabs on the shelf. Then I manually up-flicked each tab to close it and remove it. PIB. Took a while. They were gone from the shelf and stayed closed. If it shows up on the shelf, it will come back. ***Back up anything essential the first time you do this.*** After the initial purging, it becomes simple.
Place the tab links that you saved from the shelf as your backup into named groups using the new group tabbing feature. You can place many into one group. Then, just don’t remove those tab pages from the shelf. They’ll take up much less room this way. I think this just how the tabbing works now. The shelf will always be there. They will show up on the shelf, but there will be fewer, and it will be more organized.
I hope this helps.