Focusing on native English speakers from the UK, inserting an r between words is a hyper correction of a phonetic rule in British English; the final r in a word is silent unless followed by a vowel. It doesn’t appear to be considered standard, judging by the teaching material that the UK government provides for learning English.
Concerning all English speakers, you may not be able to generalize a single cause. In answering this question, I provided another plausible reason why a native Mandarin speaker may say idea-er when speaking English, Erhua. English is a language spoken by over 1 billion people as an L2. Given how many languages influence the phonology that speakers apply to English, it may not be reasonable to assume that the influences acting on a subset of the 380 million native speakers are those that also govern the verbal habits of the other 1 billion speakers.