Sentence Correction

Please help me to take a look at this. The correct answer explanation does not make sense at all!!!

Question:
Out of this season's obsession with all things political have grown a market for official memorabilia and trinkets that are flooding the shops with t-shirts, bumper stickers, and lapel pins.

According to the solution, the correct sentence should be:
Out of this seasons's obsession with all things political has grown a market for official memorabilia and trinkets that is flooding the shops with t-shirts, bumper stickers, and lapel pins.

This is what I think: There are two subject/verb agreement in the original sentence. The first subject is this season's obsession(singular), thus the "have grown" should be replaced with "has grown". The second subject is official memorabilia and trinkets, which is plural, thus "are flooding" is correct.

But the explanation said the first subject is market, we should use singular form verb. How comes the first subject is market? I don't understand. It should be "this season's obsession". The explanation agrees that the second subject is "official memorabilia and trinkets", which is plural. The correct answers eliminated all answers with "floods" because they are singular form, but the explanation thinks "is flooding" is correct", which is also singular form. This is confusing. Can someone please explain?

FYI, here's the original explanation: The sentence creates a subject-verb agreement error by ascribing the verb have to the singular subject market. Choice C and E is have a subject-verb agreement error with the usage of the singular verb floods for the plural memorabilia and trinkets.