Some people have buttons set to bet 2/3 or 3/4 the pot or whatever
In a tournament with antes, this can mean betting some odd amounts with the click of a button. IE, it's easier to bet 583 into an 875 pot than it would be to bet an even number like 550 or 600 because I can just hit the 2/3 button.
If they're not using such a macro, then they're either playing goofy/wild, or they made that bet for a specific reason. These reasons are either because they are feeling wild/playful, because they are a little scared, or because they are trying to get a call (either by making it appear cheap, or by acting weird so people call from curiosity/doubt).
Note: From my experience if someone who usually bets exactly 2/3, 3/4 etc. suddenly changes and bets an even number, it's usually a slightly bigger number than it would have been, and that is because he is trying to get more value out of his big hand.