Because BlueStacks uses "binary patching" to inject controls into games. This behavior mimics certain malware techniques. As long as you downloaded from cdn.bluestacks.com or www.bluestacks.com , it is a false positive. Submit the file to your antivirus vendor as safe.
The Ultimate Guide to BlueStacks 5.7.121: Features, Benefits, and Setup bluestacks 5.7.121 download
Choose "Performance" mode and set the Graphics Renderer to "DirectX" for older GPUs or "OpenGL" for modern dedicated cards. Because BlueStacks uses "binary patching" to inject controls