I've had a permanent metal retainer behind my front lower teeth for 5 years. Lately they wobble slightly even though the movement isn't visible. Why?

May be loose. The retainer may be coming loose on one if the attachments. There are several types of fixed metal retainers for post-ortho holding of teeth in proper position. Most are adhered to teeth with bonded resin. These can get popped off and lose seal. Go to dentist or orthodontist and get it reattached or recemented. You are at risk of loosing or damaging appliance and/or teeth shifting. .
What's wobbling? Are the teeth moving or is the retainer wobbling? Sounds like the metal retainer is either debonded or the cement has washed out. See the orthodontist to rebond or recement.