In the Marvel Comics series "Annihilation: Conquest - Starlord," Groot is introduced and speaks normally. However, in "Annihilation: Conquest," which is the mini-series immediately following "Starlord," he only says the phrase "I am Groot" repeatedly. This continues through to "Guardians of the Galaxy." The "Starlord" mini-series was written by Keith Giffen, whereas "Annihilation: Conquest" and "Guardians of the Galaxy" were written by Dan Abnett and Andy Lanning. Was this simply something that was changed between writers? Is it ever explained why Groot went from speaking normally to only saying "I am Groot"?
Apparently Groot used to speak quite a bit.
There are conflicting explanations of why Groot's speech deteriorated, so it's probably just different writers interpretations of the character. The "official" explanation (quoted from Wikipedia) is:
The mature form of Groot's species is robust and heavyweight, which causes the organs of acoustic generation to become stiff and inflexible. It is this hardened nature of Groot's larynx that causes people, who are oblivious to the subtle nuances of his speech, to misinterpret him as merely repeating his name.
However, Groot has "died" and regenerated from a sprig on several occasions, and he's had the ability to speak normally both when he was very small (a sapling?) and when he appears to be in a very mature form.
The alternative in-universe explanation is that Groot simply lost the ability to speak normally on one of the occasions where he was destroyed and subsequently regenerated.
It has not been specifically addressed, as far as I can tell.
However, we recently got solid confirmation that both his speech degradation, and his inability to grow back to full size, were abnormal states for him. In Infinity Countdown #1, the Guardians confront the cosmic elder being known as the Gardener, whose had been creating an army of vicious Groots. However, that's because the Gardener hasn't been well. Groot helps with that:
Returned to normal, the Gardener immediately realizes that Groot himself is in a damaged state:
And, the Gardener heals Groot, restoring him both to his normal stature, and his full vocabulary:
His first few incarnations are new. Groot appeared in his first comic in a time without heroes: a place long ago. Groot probably just got a bad throat becease he's been around for so long. ... Groot must be very old by now. So it must sound like I am Groot but he is actually saying proper words... Just badly.
In short: His throats was flexible but was stiffened with age