I actually don't think the issue is the front of park security, instead I think it's how the park is laid out. Here me out here.
The incident happened outside of the Musik [sic] Express. Lets go to google maps...
★★★★☆ · Amusement park
www.google.com
Right there is a gate for service vehicles to enter that a bag could be thrown over. My educated guess is drama between groups started outside of the park, either today or built up all week, and then it spilled into the park. Whenever things happen in parks like this it's always the outside world spilling in, not usually random acts of terrorism.