While it's basically a trend, there are psychological reasons that support this trend: in consumer's minds, all lowercase logos are friendlier, while proper case logos have more authority.
Across two studies, the authors find that psychologically, consumers
feel closer to lowercase wordmarks, which increase perceptions of
brand friendliness compared with the uppercase wordmarks.
Extract taken from The effects of uppercase and lowercase wordmarks on brand perceptions (PDF Download Available).
Additionally, there's evidence that lowercase text is easier to read, see article by NCBI Letter case and text legibility in normal and low vision from where I took the following image:
and this paragraph:
Miles Tinker, an authority on legibility and typography said
“Lower-case letters have more ‘character’ in terms of variation in
shape and the contrasting of ascenders and descenders with short
letters. This leads to characteristic word forms that are much easier
to read than words in all capitals” (M. Tinker, 1963 p. 34) Tinker
found that while upper-case text was perceived at a greater distance,
it had a ‘retarding effect’ on reading speed, especially for long
intervals of reading, and was preferred by only 10% of readers,
compared with 90% for lower-case text. (M. A. Tinker, 1932; M. A.
Tinker & Patterson, 1929)
While there's a lot of trend and imitation, there seems to be some solid arguments that support this trend from a psychological point of view