While every player has "bad luck" stretches, you are right, it is not an excuse. The fact that JBJ's bad luck stretch fell during a stretch of severe struggles at the plate just made it worse. Even, if he had some better luck, he'd still be at about .220/.650.
Who knows why this pattern seems to repeat itself every year. If it was easy to answer, maybe it could be solved.
I also feel like his hot and cold streak have been somewhat exaggerated. Iknow some of his hot streaks were straddling 2 months, making both months look normal, but these numbers do not lie:
Months that JBJ's monthly OPS since Aug 2015 was...
3- 1.000+
0- .840 to .999
5- .770 to .839
3- .677 to .736
5- .596 to .651
1-- at .517
8 months over .770
3 months .675 to .770
6 below .651
11 out of 17 months, he's been over .677. It seems like he's had more bad months than this.
He actually has had more decent, good and great months than bad ones at almost a 2 to 1 ratio. I believe, this year has seen his longest bad streak since pre-2015.