An Indo-Pak Sufi sect call Muslims outside their group Wahhabis because of Muhammad ibn Abd al-Wahhab as he called many of their practices innovations with no basis on Qur'an and Sunnah. Any Muslim who disagrees with them is called Wahhabi in a negative way even if they follow the above scholar or not. More seriously Al-Wahhab is a name of Allah so to use it negatively is mocking Allah. So should they use such a term? They also use the word wobbler in a similar context.
Shortly this is a kind of backbiting or gheeba (for more details read Is there a hadd punishment for gheebah/backbiting? Please Answer) or mockery.
Allah the almighty says:
O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers. (49:11)
It is a sinful act to call other Muslims whom apparently follow the orders of Allah or believers by nicknames they might hate or dislike no matter if this may apply for them or not (also read here in qtafsir an interpretation of the verse based on a summary of tafsir ibn Kathir).
Note that there's no connection between al-Wahhab the supposed name of Allah and the nickname "wahhabi".
For further information you should read the verse above in more context (for example 49:9-17).
Side note on "Wahhabi": meaning and history
Note that if one wanted to refer to a follower of Muhammad ibn 'Abdulwahhab correctly than calling him "Wahhabi" in Arabic is wrong, as Abdulwahhab (the father of the Scholar) has nothing to do with the movement of his son, a more correct reference would be calling him "Muhammadi".
Historically the first whom were called "Wahhabi" where the followers of Abdul-Wahhab ibn Rustum the leader and founder of the first Ibadi Dynasty in what we now call Algeria and Tunisia.