You're talking about what's called "terminal r." It's a linguistic phenomenon which happens naturally in non-rhotic (or r-less) speech. Most British English today (maybe all of it) is non-rhotic. On the other hand, Canadian English and most American English is "rhotic" (or r-colored).
In the few places in the U.S. where non-rhotic English is spoken, it is not unusual for speakers to tack an 'r' on the to he end of a word ending in 'a' like the late President John Kennedy's famous "Cuber" for Cuba. I hear New Yorkers on the radio all the time pronouncing words like drama, trauma and idea as dramer, traumer and ideer.
It is interesting to note that in interviews, Britishers (or Britons, Brits etc.) claim that they are unaware that they pronounce words ending in the vowel 'a' with an 'r' . I believe them. Nevertheless, Americans always do notice it because they don't pronounce these words with a terminal r.
The change in England from rhotic to non-rhotic pronunciation began in the eighteenth century, maybe the late seventheenth century at the earliest. It took a long time for it to spread. America was colonized largely before most people in England began speaking non-rhotic English. Therefore, it's a rare sound in the U.S.
The theory as to why non-rhotic pronunciation exists in Boston, New York and Philadelphia (Philadelphier) is that American colonists in these cities were in closer contact with the mother country than American colonists elsewhere in the colonies through maritime trade and commerce. Therefore, they were affected by linguistic changes going on in England that American settlers living in Appalachia or the South were not affected by.
After the Revolutionary War (1783), the United States was cut off almost completely from England and thus American and British English started going their separate ways.