It sounds like you have used ground/mince beef that was too fatty and the fat has rendered out into the casserole. I tend to avoid low fat beef as we want plenty of healthy fat to keep us fuller for longer and all the benefits that come from a high fat diet, but I also avoid the highest fat beef as too much fat comes out in the cooking process for a dish such as this one.
On a ketogenic diet, your entire body switches its fuel supply to run mostly on fat, burning fat 24-7. When insulin levels become very low, fat burning can increase dramatically. It becomes easier to access your fat stores to burn them off. This is great if you’re trying to lose weight, but there are also other less obvious benefits, such as less hunger and a steady supply of energy. This may help keep you alert and focused.