If there's betting after the start its a very rare malfunction of the tote system. But conspiracy theorists and haters believe the nonsense.
As someone who has worked around IT and finance my whole like, I am skeptical bets are placed after the race starts. What is happening is the final pools are not updated instantly. If large dollar amounts of bets are being placed after the race starts it is major fraud and would be easy for an IT auditor to find. Either there would be bets with timestamps after the races started or there would be code which which changes the timestamp and that code would only called for certain bets.
Since there are a number of people on Horseplop who are sure this is happening, I suggest they start reporting this to the FBI and the state attorney general of any state which has a track involved.