is/are

"Is" is the correct form of the verb to use. In your sentence, "a concern" is a predicate nominative which "renames," which describes the subject, and the subject is "things being thrown from the stands." The verb "is" agrees with the predicate nominative ("a concern") in number--it is singular.

Re: is/are


As you point out, it is an awkward sentence and so most journalists/editors would avoid the difficulty by rewriting it.

"Things being thrown from the stands is a concern" might be rewritten as

"The problem of objects being thrown from the stands is a concern."

Thanks, Mc Tag.