Why do my udev rules run if I use udevadm trigger, but NOT at boot time?

I actually have a way that will wait just the right amount of time, not an arbitrary 30sec. I did it on Raspberry Pi to automatically mount all connected USB disks upon connection but also on boot time.

The rule is similar to yours:

$ sudo cat /etc/udev/rules.d/10-usb_automount.rules 
KERNEL=="sd*", RUN+="/home/pi/bin/usb-automount"

Now the script is actually a recursive call (and I know this is evil):

$ cat /home/pi/bin/usb-automount
#!/bin/sh

ROOT_RW=`mount | grep 'dev/root' | grep -E '\(.*rw.*\)'`

if [ -z "$ROOT_RW" ]; then
 sleep 3 
 /home/pi/bin/usb-automount & disown
else
 /home/pi/bin/usb-automount.sh
fi

Note that the "grep 'dev/root'" is specific to Raspbian OS, so on Ubuntu you will need to design your own grep to detect the rootfs (or even better design some universal grep). Notice that the script will call itself in the background and exit and only if the rootfs is "rw" will call the right mounting script. The script "/home/pi/bin/usb-automount.sh" does the actual mounting or in your case the logging.

Note this script still takes 3 seconds to execute, so you may further optimize by changing to:

if [ -z "$ROOT_RW" ]; then
 ( sleep 3; /home/pi/bin/usb-automount ) & disown
else
 /home/pi/bin/usb-automount.sh
fi

However I never checked that and do not know if this will work as expected (I am no scripting guru).

As mentioned by others, the processes started by udev's RUN= directive should be short-running. I would like to suggest another simpler way of decoupling long-running process from udev by using system scheduler at command:

ACTION=="add", SUBSYSTEM=="pci", ATTR{vendor}=="0x8086", RUN+="/usr/bin/at -M -f /sayhi now"

Just make sure that your /sayhi script is /bin/sh compatible - this is the shell that at uses.