I believe it will be because epoch time is calculated as the number of seconds since 1st Jan 1970. The two dates that are not being converted occur before this time, so the value would be negative, which is not supported by Splunk. What are you trying to achieve by converting the timestamps to epoch?
Does this happen when you open the CSV in excel, or was the CSV saved from excel? Sometimes it does some weird auto-formatting stuff on dates ...
Do you just want to reformat it as YYYY-MM-DD?
If so, you could try just using the replace command instead of fieldformat:
| replace "* 00:00:00.0" with "*" in DATE_OF_BIRTH