I would have thought there should be bolts fitted in those holes!
Overfilling and excessive splashing, or wrong grade oil could be part of it. Maybe those bolts, if fitted, need suitable washers, tightening, or thread sealant?
I have heard of (increased) oil leakage/seepage from older tractors which have been run with thinner oils.
For instance, there is far too much oil lost from the fan shaft on one of my tractors but that may be due to increased porosity of the replacement bearings fitted some years ago, or the lighter grade (and detergent type) of oil used.