Injectors fail over and over

I have a 1987 Integra. The fuel injectors started failing after 170,000 miles so I had them replaced. About a month later, one of the new ones failed. The shop replaced it for free. Two weeks later another one failed. The shop replaced it free as well. The shop then informed me that is sounds like something else might be the problem and would start charging me for the work. What could be causing this or am I getting screwed?