This is a thing that I noticed while preparing for an oral examination in my Japanese class.
Japanese and Korean definitely share a lot of words due to the heavy influence of Chinese on both languages. There is a very clear pattern of sound changes as you go from Korean's Chinese loanwords to Japanese's Chinese loanwords. Even some non-Chinese loanwords in both languages sound the same (e.g. 나란하다 (naran-hada; "to line up") and 並ぶ(nara-bu; "to line up") *although the Japanese does contain Chinese character in the writing, it is a meaing-reading in Japanese which means the word itself is not borrowed from Chinese but only written with Chinese character.). Due to such lexical proximity and grammatical similarity between the two language, it tends to be easier for me to switch between Japanese and Korean than to switch between Korean and English or between Japanese and English.
What I noticed today, was that, such tendency also happens between Mandarin Chinese and Japanese. My friend who prepared the Japanese oral exam as partners with me mistakenly dropped down some Mandarin while discussing our script.
It is true that it is just one instance of it, but I feel a much stronger connection between Korean and Japanese and I think that happens with my friend between Chinese and Japanese.
So wouldn't it be possible to speculate that closer languages are more closely coded in the brain. It is simply much easier to drop some Korean among Japanese words than it is to drop Korean words in the middle of English words.