Oh man, I was born to answer this question. Syntax refers to sentence structure broadly and is the right term here. "Frogs and cranberries? Must be Fall!" was the intended meaning, but the writer left out all punctuation. In combining the two phrases, they accidentally made "frogs and cranberries" the predicate nominative of the linking verb "must be." Predicate nominatives are used to rename or redefine the subject of a sentence, as in "he must be an athlete," or "my name is Bob." In this type of sentence, the two nouns refer to the same exact person/thing, but one specifies the other further. Bob literally is your name, he literally is an athelete. So it implies frogs and cranberries literally are the same thing as fall. There's not really a name for this error other than "syntax," because sentence structure is at its heart. This is nonsensical.

Second, even if it were written correctly, it would still be funny because neither cranberries nor frogs are on the cultural radar of "things associated with fall"; you'd only associate them with fall if you were a specific flavor of nature/ecology nerd. Peterson Field Guides posted this first (unironically), and it became popular in the Wild Green Memes for Ecological Fiends group, and followers of either page are likely to be exactly this kind of dork. It implies that the writer is dorky/out of touch enough to believe that frogs and cranberries are right up there with pumpkin spice on the list of "must be fall!" signifiers. This is endearing.