It could be many things, not just electrical issues that caused it. One likely candidate might be thermal issues? Is the sufficient airflow in that part of the rack - i.e is something blocking/disturbing the airflow? Is some other device that's running hot blowing onto the BIG-IP? Another far less likely possibility might be vibration.
The other possibility - just bad luck. Initial device failed - replacement device may have been damaged in transit just badly enough to boot up the first time, but fail after hitting some threshold (e.g. temp?)