The triglyceride response to GLP-1 receptor agonists is actually one of the most robust and consistent findings in the clinical trial data. The SUSTAIN and STEP programs both showed significant TG reductions, and mechanistically it makes sense — GLP-1 RAs reduce hepatic VLDL production, improve insulin sensitivity (which reduces de novo lipogenesis), and reduce free fatty acid flux to the liver.
In the OP's case, the 45 lbs of weight loss is clearly the dominant driver, but there's likely a direct hepatic effect from semaglutide on top of that. The VLDL reduction of 60% mirrors the TG drop perfectly, as expected since VLDL is the primary triglyceride carrier.
Given the improvement, I'd recommend repeating the panel at 12 months to confirm stability, and asking about an ApoB level to get a more complete picture of residual risk.