From Wikipedia - linking R and intrusive R
In many non-rhotic accents, words historically ending in /r/ (as evidenced by an ⟨r⟩ in the spelling) may be pronounced with [r] when they are closely followed by another morpheme beginning with a vowel sound. So tuner amp may be pronounced [ˈtjuːnər æmp].[nb 1] This is the case in such accents even though tuner would not otherwise be pronounced with an [r]. Here, "closely" means the following word must be in the same prosodic unit (that is, not separated by a pausa). This phenomenon is known as linking R. Not all non-rhotic varieties feature linking R. A notable non-rhotic accent that does not have linking R is Southern American English.
The phenomenon of intrusive R is an overgeneralizing reinterpretation of linking R into an r-insertion rule that affects any word that ends in the non-high vowels /ə/, /ɪə/, /ɑː/, or /ɔː/; when such a word is closely followed by another word beginning in a vowel sound, an [r] is inserted between them, even when no final /r/ was historically present. For example, the phrase bacteria in it would be pronounced /bækˈtɪəriərˌɪnɪt/. The epenthetic [r] can be inserted to prevent hiatus, two consecutive vowel sounds.