Check your rear shock mounts...

:
2015 Mazda 6 -- Skyactiv 2.5L NA W/ 91 Octane Tune
If your car is older, higher mileage, especially if living in a rust belt area, be sure to inspect them.
 
I had one of these break on an old 2006 Mazda3...I was going slow at the time but not a pleasant experience!
 
Even if they haven't failed per say, if they are old, wouldn't hurt to replace, as the rubber in them gets old.
 
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