The three-month period ending July 31st was the wettest in Illinois history:
Illinois experienced its wettest May – July on record with 500 mm of precipitation, 200 mm above the 20th century average, according to the National Centers for Environmental Information. Most of that was due to the record precipitation of June with 240 mm statewide, based on their latest numbers and discussed in more detail here.
That is about an extra two months of precipitation during that three-month period.
Factors include the Ridiculously Resilient Ridge out west and possibly the incipient El Niño taking hold in the Eastern Pacific.