This is caused by the insect apocalypse.
It's because the harm caused to the bees is from factors that strike as well other insects: mainly pesticides, and global warming.
Honestly it's such a dirty metric with so many compounding variables I'm shocked that anybody is seriously considering it as a meaningful data point beyond the most basic, surface-level understanding of the issue. To be honest the whole thing reeks of the usual hacker-news crowd issue where people want to pretend they know about a topic (e.g. systemic issues in pollinator/agricultural pest population management in high-intensity modern industrial agriculture) but are underestimating the complexity of the issues by a huge magnitude.
You know, it is just a matter of knitting all of our knowledge bits together. Lots of nasty chemicals in the environments, especially design to harm insects. Add to that anormal temperatures and rain levels.