Yoda's unique speech consisting of reversed grammar is iconic.
Is there any reason he has adopted this speech pattern?
All other alien species encountered seem to have no trouble speaking normally. Why does Yoda speak in this way?
In universe, we don't know much about Yoda's species mostly because "George Lucas maintains a strict policy of keeping the history, name, origin, and whereabouts of this species secret".
Yoda spoke an unusual version of Basic. He usually tended to place verbs (especially auxiliaries) after the object and subject (an object-subject-verb format). An example of Yoda's speech pattern: "When nine hundred years old you reach, look as good you will not." Most agreed that this pattern of speech was convoluted, and while it seems as though others of his species (e.g. Yaddle) had the same penchant for rearranging sentences, not all of them did (e.g. Vandar Tokare).
So, since others (but not all) of his species speak this way, it's probably a cultural matter.
Presumably, English (or the equivalent in the Star Wars universe), isn't the first language of Yoda's species.
Speakers of 2nd and 3rd languages to their mother tongue often struggle, sometimes permanently, adjusting to new grammatical conventions in other tongues.
Many, just as we see with immigrant populations on earth, adjust well and manage to speak fluently like natives. Other individuals will learn just enough to get by and never bring their language level past a certain level of polish.
Presumably, when Yoda was learning the language, it wasn't done in a setting that focused on grammar and proper syntax. Perhaps he learned it by ear, or taught it to himself in isolation.
“Does she speak backwards? I’m like, ‘No, I don’t think so. I think that’s a Yoda thing. Frank Oz told me once that Yoda speaks that way specifically in honor of his own master. That was what he had thought about it. I try to keep moving forward these thoughts. And Bryce on her own made a great Yaddle.”
The implication here is that Yoda is perfectly capable of speaking Basic with the correct syntax, but for his own reasons chooses not to.
This is confirmed in the (canon) Yoda #7. He can speak Basic perfectly well.