What is an incidental retroaortic left renal vein? The key term here would be "incidental" as it appears that something caused it to be retroaortic

Normal finding. The left renal vein which comes off the inferior vena cava normally crosses in front of the aorta and not behind it in the usual anatomy. In about 5% of people the left renal vein will pass behind the aorta (retro aortic). This is a normal anatomic variation and normal for that person. It is usually identified on ct.
Incidental. Means it was noticed but does not have an implication on the other findings or symptoms of the patient, basically it is unusual but harmless.