Non-rhotic dialects tend to drop "r" sounds, so why is one added here when there is no "r" in the word?
It is called the intrusive "r" by linguists. It is a regionalism present both in England and in the U.S. The following extract from Grammarphobia has the story:
This “r” business is a bit more complicated than you might think. The short answer is that the addition of an “r” sound at the end of a word like “soda” or “idea” is a regionalism and isn’t considered a mispronunciation. Here’s the story.
In English words spelled with “r,” the consonant used to be fully pronounced everywhere. But today, some speakers (particularly in certain parts of England and the Eastern US) give “r” its full sound only before a vowel, either in the same word or in the word immediately following.
Take the word “better” as an example. These speakers pronounce it bettuh, but they add the “r” if a vowel sound follows. For example, the same speaker would say, “I’m bettuh paid,” but “I’m better off.”
To use another example, the same speaker would say, “New York isn’t fah,” but “New York isn’t far away.” Such speakers are basically using the “r” to link what they pronounce as two vowel sounds. Linguists have called this phenomenon the “linking r.”
Because of the tendency to pronounce an “r” when it occurs between vowel sounds, many of these same speakers go a step more and add an “r” where it doesn’t belong, once again between two vowel sounds.
Linguists call this the “intrusive r,” and that’s what you hear when someone says, “My soder [soda] is flat.” The “r” is inserted AFTER words ending in “uh” sounds just BEFORE words starting with vowels. So the same speaker would say things like this:
(1) “My old sofa died,” but “My new sofer is great.”
(2) “That’s a bad idea,” but “That idear annoys me.”
(3) “England and America joined,” but “Ameriker and England joined.”
(4) “The law says so,” but “It’s the lawr of the land.”
(5) “Tuna grills nicely,” but “Tuner is my favorite fish.”
- This explanation of the “linking r” and the “intrusive r” is greatly simplified. Some speakers will add “r” more generally, pronouncing “idea” and “soda” as idear and soder even at the end of a sentence. And some speakers will drop “r” more generally, saying things like bettuh and fah even before vowels.