Getting many requests with User Agent [Mozilla/5.0]
It looks very likely that these are some sort of bot, as that user agent is not used by any mainstream browser.
It will be worth you filtering your logs to extract just these entries, and checking if they are following any sort of obvious bot-like pattern. For instance, you may see:
- A request every X seconds exactly
- That they all happen at a specific time of day
- That they all happen within a very short period of time
- That they request URLs in alphabetical order
- That all the requests come from a single IP address, or limited range of IPs