Recently, I've seen that Gmail and Dropbox have changed their notification icon and have started using bell.
I would like to understand:
- Why bell?
- Is it becoming industry standard?
On that premise, it's easy to go for a bell when it comes to notifications, as they are just that; signals that something is happening.
A bell requires - maybe even demands - your attention, and so does a notification!
As to whether it's industry standard or not, it currently doesn't pass a quick Google test*, but as mentioned by others, Gmail's/Dropbox's adoption of this pattern might help.
On top of that, it doesn't necessarily have to be industry standard to be the right icon.
Edit: *if you alter your search to the word "alarm", which might be less contaminated, the result does get a lot closer.