This in fact could be a power issue as the output current of the I2C output is limited and if this limit is surpassed, it could make the bus crash. So, the first thing I would try to use an external power supply to power the I2C devices.
I believe the reason why the pull-down resistors seemed to improve the bus' behavior is because you were decreasing the charge exposed to Edison's output. However, when you started to add additional LED backpacks you increased the charge again and you might have reached the current limit again.
Try using an external power supply for the LED backpacks and let me know if the issues persists, I'd be glad to help if that was the case.
Thanks Peter for your elaborate response i appreciate that. Yes, i am powering my I2C devices from an external power source which can provide enough current to all I2C devices simultaneously, and current limitation doesn't seem to be the problem as devices are not showing any sign of dimming as well.
I understand, in that case, it would be interesting to see the I2C signal the Edison is outputting. Do you have access to an oscilloscope or to a logic analyzer? Comparing both the output before the I2C bus crashes to what happens when it crashes.
Have you tried verifying these signals?