Loose teeth.. why?

I m 20 and all my teeth (including buttom wisdom teeth that just came out) have an approximately 1mm mobility... my gums are healthy and I have no cavities (checked with dentists). No bruxism or root resorbtion either. I had braces for 4 years, took them off 2 yrs ago and worn a retainer for 1 year. Dentists said that might be the cause. Id like to hear more opinions... thank you in advance.

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