^^^this.
...also, if this had happened to a team that was playing very well it probably would not have been made an issue. But just like when the chicken/beer thing occurred, and even though it wouldn't normally be considered "a big deal", it coincided with a time when the Sox were already losing and in disarray... and in a big market like Boston, it was magnified even more..
...and only a week or so ago John Henry gave both Cherington and Ferrell a "vote of confidence"...and as we've seen in sports before, sometimes that's ultimately a "kiss of death"...since the vote of confidence, the Sox have gone 5-10.
...if the Sox don't play .500 ball or better for the rest of the month, I think Ferrell could be let go.
...and the linked article by Edes in the OP also explained why this incident "could cost manager John Farrell dearly".