the letter doesn’t say, and the reporter may or may not have had good reason. we don’t know based on the information provided.
the fact that a report was made does not inherently mean that abuse or neglect was taking place, only that someone reported concern. the fact that the report is being investigated does not mean that abuse or neglect was taking place, only that someone with CPS agreed to open a case based on what they were told.
i could call CPS and say that you are abusing a child or other vulnerable person, provide enough information about you and a plausible concern (in theory at least, whether it’s based in fact or not), and CPS could choose to follow up on that report. i can make this report and they can investigate regardless of whether there is any actual evidence of abuse or neglect.