That would be my 'guess', a faulty model.

The bounding box you normally see in WED is just so you can see the models
maximum dimensions, it doesnt actually exist.

Try testing by putting your player-start position inside the box in a
valid place and see if everything 'inside' collides ok.

If it does, there may be some hidden bounding box built into the model.
Open the model in MED, and zoom the 3d view right out. Any bounding boxes
will show here if they exist.

If you know how to post files here, post an untextured copy of your park model.