maybe albaby1 can explain the "grand theft" charge or why he was free after two felony charges in less than a month....

As for grand theft, that's defined by California penal code as any theft with a value greater than $950. Per the article, he was ripping signs off of storefronts. That can either be mere property damage (tearing them off for no reason) or theft (tearing them off to keep them) - and since the former is probably a lesser included charge within the latter, you charge with the higher offense until you know what his motives were. Similarly, I expect that the arresting officers didn't know exactly the value of the signs, so it might be their policy (just speculation here) to charge with grand theft rather than petty theft until they know the facts. Since one policy goal behind the charges is to inform the defendant of what they're accused of, it can be the appropriate call to overcharge at the initial stage and then just reduce charges later if the value turns out to be lower.

As for the other charges, again this is just speculation - but prosecutors probably couldn't have proven that this guy needed to be incarcerated prior to trial. You're presumptively innocent of crimes, so until you get convicted of those crimes you have to meet certain criteria for pre-trial detention. Depending on his history and what the felony charges were, prosecutors may not have been able to establish that.