Perhaps you have a drop? Another possibility is that if you are on shared hosting, your neighbors could have caused trouble. Your IP reputation is damaged, and you are affected indirectly. However, this rarely happens. Open “Security Issues” in GSC and check the details. And check it with different scanners like VirusTotal — maybe there really is something you can't see. Sometimes a malicious script or plugin gets pulled up. Usually, after a review, they remove it if everything is really clean. You'll have to wait a week or two at most.