Backdoor attacks are an important safety risk for large language models (LLMs), yet existing studies often rely on manually crafted triggers and fixed poisoning pipelines. This limits the diversity of attack behaviors and makes it difficult to systematically evaluate defense robustness. We introduce AutoBackdoor, an agent-driven framework for scalable LLM backdoor red-teaming. Given a target scenario, AutoBackdoor automatically generates triggers, constructs poisoned samples, refines them through self-feedback, and produces backdoored models through fine-tuning. We evaluate the framework across four realistic scenarios: Bias Recommendation, Hallucination Injection, Error Code Generation, and Peer Review Manipulation. Experiments on LLaMA-3, Mistral, Qwen, and GPT-4o show that AutoBackdoor can achieve over 90% attack success with only a small number of poisoned samples. We further find that existing defenses provide limited protection against these agent-generated attacks. These results suggest that automated red-teaming can serve as a useful tool for stress-testing LLM backdoor defenses and understanding emerging risks from agent-driven model customization pipelines. Our code is available at https://github.com/bboylyg/AutoBackdoor.