What was the reasoning for turning off the fences?

Still it wasn't his goal to avoid any disaster, rather on the contrary.

In the end Dennis Nedry was hired by a competiting organization/company for industrial espionage, to steal the embryos. So producing some disaster by turning off the fences could on the one hand help him escape unnoticed or at least unbothered. And on the other hand sabotaging the park could only be in the best interrests of his real employers.

Of course he didn't anticipate that it could also make it much more difficult for him to escape either. Maybe he didn't even anticipate anybody getting seriously injured (or he just didn't care, but "even Nedry knew to keep his fingers off the raptor fences"). From his point of view a little chaos could only work to his advantages (even if he was wrong in the end).