False sense of entitlement
Someone might feel entitled to your money and decide to help themselves. For example, if you have adult children they might view your money as their inheritance and feel like they can dip in and take money early.
There could be a gradual change in behaviour, where you put someone you trust in charge of your financial affairs and they initially look after your money responsibly, but gradually start taking advantage.
It could be someone in a position of trust - like an attorney or person you’ve appointed to manage your affairs – who is not up to the task.