Outside of congenital issues, what can cause chiari malformation? Had a good MRI a year ago, and now it’s lowered 3-4mm

Trauma ? Infection? By definition, a Chiari malformation IS a congenital condition whereby the lower parts of of the brain and brainstem extend below the foremen magnum. Suggest you discuss with a Neurosurgeon for more details. It's possible trauma can create a condition similar to this, but that would not be considered a 'malformation' .
Chiari. malformations are by definition congenital. If your cerebellar tonsills are "lowered" 3-4mm and you are otherwise asymptomatic and with out other MRI findings, it is normal. Since it is normal, it may not have been mentioned on the prior MRI report. If there has been an actual real change, someone needs to compare both MRIs and find out why.