The police arrived suddenly, and without warning started arresting everyone without wanton disregard for whether the individuals being arrested had done anything wrong. That much is agreed to by both witnesses and the police.
The problem is, that in and of itself is bogus. According to an op-ed piece published in the Houston Chronicle and written by Christine McDonald, the police had been contacted on numerous occasions on complaints of noise and general rowdiness caused by an ever increasing crowd (as mentioned by Houston City Councilman Bert Keller on KHOU-TV).
Even if warnings had been given to disperse from that lot on other occasions, and even if a warning given days before could be held to still be applicable (which would probably only be the case if the store owners had requested that the people being dispersed never come back to shop in their store again), the police arrested everyone without regard for whether individuals had been present for any earlier warnings. Certainly one cannot be held liable for failing to heed a warning one was not present to hear.
It may be that some of the people who were arrested deserved it. It could even be that most of them did. Even if 2/3 of the arrests were legitimate, however, that would in no way excuse the cops' deliberate harassment of 90+ people whom there was clearly no cause to arrest.