what probably occured is that you had auto generate collision set to enabled when you imported the mesh and it created a large rectangular collision volume. to verify this open your mesh in ue4 and enabled the showing of the simple collision by clicking collision near the top then selecting simple. this will show you the shape of your collision. now to fix the issue you will need to create your own custom collision in a 3d modeling software, or disable collision on the mesh and add in some collision volumes of your own in the level such as a few box collisions.