If you're talking about Google Ngram data, it's mainly a combination of:
Insufficient data from the nineteenth century, meaning that even if there's only a few sources where "bruh" is used that'll be enough to skew the results;
Technical issues where unrelated words are read as "bruh" because Google can't read books that old as competently, e.g. in this 1881 edition of Shakespeare we have Google reading "Bru." (an abbreviation for Brutus in Julius Caesar) as "bruh".
If we look at the actual data from Google Books, most mid-nineteenth century sources that use the word "bruh" (and which aren't misreadings) are mostly either:
Dictionaries. In the absence of the International Phonetic Alphabet (invented in the 1880s), English-speaking dictionary makers used "bruh" to approximate the pronunciation of the French syllable bre [bʁə] in words like brebis "ewe" (female sheep), or of the Hindi syllable ब्र [bɾə] in words like Brahma. Since dictionaries would use dots or hyphens to split the word into syllables (e.g. bruh-bee for brebis or bruh-ma for Brahma), Google counts these as a use of the word "bruh."
References to the southern pig-tailed macaque, a type of Indonesian monkey. In the nineteenth-century, Europeans called this monkey the "bruh monkey." This probably comes from Malay or Minangkabau, where beruk (Malay) or baru (Minangkabau) refer to a specific type of monkey.
Merriam-Webster says the etymology is Malay, but I'm partial to the Minangkabau option because 1) you'd have to explain away the final -k in beruk if "bruh" did come from Malay, and 2) the "bruh monkey" seems to almost always be referenced in relation to Sumatra, large parts of which is Minangkabau country.
There's no connection to the slang word "bruh" as in "brother".
The "spike" isn't really much of a spike when you look at the percentages. If you examine some of the books from that period they include wildlife collection directories featuring Bruh macaques, publications of folk stories with "Bruh" characters (later "Brer", as another nGram search will show you), Hindi dictionaries, and directories containing the surname "Bruh".
I'm not aware of any specific academic study into the prevalence of the word but it was a time when the British colonisation of India and the European colonisation of America were well under way - both activities led to a wider awareness of "Bruh" as a word and a title.