Overall attacks are higher in summer because of the number of people.
Here is a good study from Herraro who is the world leading researcher on the subject.
https://www.nature.com/articles/srep20552
Quote:
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From highest to lowest, the five most common human behaviours occurring at the time of an attack were (a) parents leaving children unattended, (b) walking an unleashed dog, (c) searching for a wounded large carnivore during hunting, (d) engaging in outdoor activities at twilight/night and (e) approaching a female with young.
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https://wildlife.onlinelibrary.wiley...0.1002/jwmg.72
An abstract from herraro on black bear fatalities. 88% of black bear fatalities are due to predation.
https://www.jstor.org/stable/3873003?seq=1
And this one on BC bears found that 79% of Grizzly attacks were females which you can infer to be defending Cubs.
The patterns seem to be Grizzly attacks tend to be about protecting young or carcasses and black bears tend to be predatory.