CAUSE
You selected two or more spaces to the left of your selected word.
Word defines any spaces before the first space to the left of a word
as part of the preceding word. Word defines the end-of-sentence
punctuation as a separate word. When there are two or more spaces
between two words and you select more than the space to the left of
the word, Word automatically selects the first word (or the
end-of-sentence punctuation), too.
This behavior is by design of Microsoft Word.
For example, in the sentence below, Word selects ". This " if you
select both spaces preceding "This", whereas Word only selects " This
" if you select only one space preceding "This".
This is a sentence. This is another sentence.
NOTE: There are two spaces between the two sentences. Word defines the
period as a separate word, so when you select its trailing space, Word
selects the period as well.
As another example, if you select more than the first space to the
left of "Apple" in the sentence below, Word selects "Fruit" as well.
Fruit Apple
NOTE: There are four spaces between "Fruit" and "Apple".
In contrast, Word does not select the preceding word or the period at
the end of the preceding sentence if there is only one space between
the sentences and you select that space along with the first word of
the sentence.