Is it against the rules for the excavation hardware to interact with the sides of the sandbox?

No. This is a change from last year. The intent is that bumping the side does not cause a team to be disqualified, but the excavation hardware cannot be designed to require a sandbox in order to operate. For instance, consider the scenario where an autonomous excavator on the moon does not have a structure (like a sandbox) to depend on for boundaries, but will occasionally have to deal with one anyway (i.e. a steep incline or a very large boulder). Please keep in mind that unsafe activities, including damage to the sandbox will result in early termination of the excavation attempt.