There has been a LOT of uneven baseball this month by a lot of teams.
I think most of the current playoff contenders have been swept this month at some point.
Most recent series sweeps against current divisional and wild-card leaders (all 3-match series except as noted):
Orioles - 26th-27th June vs Angels (2-match series)*
Rangers - 3rd-5th July at White Sox
Giants - 27th-29th July vs Dodgers
Reds - 6th-8th August at Brewers
Yankees - 20th-22nd August at White Sox
White Sox - 31st August-2nd September at Tigers
Athletics - 3rd-5th September at Angels
Braves - 10th-12th September at Brewers
Cardinals - 10th-12th September at Padres
Nationals - 14th-16th September at Braves
*most recent time they were swept in 3-match series was 18th-20th June at Mets