I think it is caused by consumption of medium chain triglycerides (MCT).
Most days around 14:30 to 15:30 I consume a snack consisting of a small amount of fruit (~40g) together with around 50g of coconut cream and/or heavy dairy cream, which are rich sources of MCT.

Here is a quote about MCT from P&V's book, The Art and Science of Low Carbohydrate Performance:

"MCT have three very important distinctions:
First, they are absorbed much more quickly [than long chain triglycerides (LCT)]
Second, MCT don't get stored in fat cells, so once consumed they need to be processed immediately.
Third, MCT are promptly oxidised in muscle cells or used by the liver to make ketones.
Thus depending on the dose, ingestion of MCTs can result in rapid elevation of ketones.
Natural sources of MCT are dairy fats (butter and cream) and coconut oil, so you may find your ketones go up more after ingestion of these foods.
About 10-15% of the fat in butter consists of MCTs and about two-thirds of the fat in coconut oil is MCT."