Bees aren't the only pollinators by a long shot, they're just the most commercialized/domesticated one. Take corn/maize, you have to rely on a pollinator speices or manual pollination if you grow small amounts in your garden but if you grow acres and acres of it wind alone will sufficiently pollinate it.
The plants still produce the pollen though. Pine trees for example produce copious amounts of pollen but it offers a relatively poor protein source so most pollinating insects don't make use of it.
Bees consume pollen as one of their food sources though, they just happen to pollinate plants due to accidental transfer while collecting pollen for their own use. You can then harvest some of the wax and honey as a commercial product and it's much easier to load up box after box of honey bees on a truck to transport to the next field than it is moths, butterflies, beetles, mosquitoes etc. Modern farming outright relies on domesticated bees, domesticated bees aren't remotely 'natural' though and nature has plenty of other means of spreading pollen sufficiently for reproduction.
A recent figure puts just domesticated honey bees at 80-100 million colonies worldwide. An average-size colony may bring in 100 pounds of pollen in a season so 8-10 billion pounds of pollen annually.
If you saw just a 10% die off of those domesticated colonies, you're talking about an insane amount of pollen. 2000 maize pollen grains weigh one milligram, so 907 million grains per pound for maize.
So the NAB scale, depending on the type of pollen, 5-89 grains per cubic meter of air is moderate and 20-1499 grains per cubic meter is a high pollen count.
Yeah, there's a LOT of air but you can see how even a small reduction in colonies could drastically increase average pollen counts. Add in global warming which can extend the growing season and open up new regions and you also see an amplification of pollen during a given period.