Because some of the purchases on your site have been marked as fraudulent by the payment platform. This happens when a user buys using stolen credit cards or Paypal accounts and the owner reports the purchases.
Streamloots implements different mechanisms to try to detect these cases before they occur but these mechanisms, despite being in continuous evolution, are not 100% infallible.
Money can be frozen for up to 180 days pending a claim on payments. If after this time the money has not become disputed, it will automatically pass to your Balance and you will receive it in the next billing period.
Checkout our Fraudulent payments and disputes article