You can't get an exact answer as to why but if the pattern is every second and it repeats so hard that it becomes impossible to do it manually then it is a bot but what is the reason behind doing it?

 - Price manipulation: Sometimes a lot of fake buy and sell orders are created to affect the price.
 - Trading strategy: to test a certain point, so if it is not achieved, the position will be closed.
 - Swing trading: bots are characterized by speed and sometimes the differences between trading pairs are profitable.
 - Technical error: Not likely, but if it is rare, it may be.

There may be other reasons.