Hi everyone,
I did similar search for April thru June 2023 and got a little bit fewer reports. This is a crude assessment but it seems that there are regular numbers of bumblebee for naturalist to observe locally.
You can do similar type of search using eBird.
Numbers of any taxon in a specific spot can rise and fall under normal circumstance. Of course it's impractical to survey your neighbors on what what they are doing in their backyards that could affect any population of critters in the neighborhood. I thinking about pesticides, landscaping, construction, cats, etc.
-Don Boucher
Corvallis