Cisco 3750 Issue

I have a stack of 6 3750's in my MDF. Additionally, I have 1 additional 3750 in an IDF on another floor, connected via fiber/SFP to a gig port on each end. When I initially connected the fiber in the IDF, it reset the entire 3750 stack, which bounced 150 phones on my main floor. Last week, the power went out on the other floor and when it came up, it bounced the whole main stack again. I didn't put it together until it happened again.

Anyone have any ideas why it would do this when I connect the IDF switch?
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I suspect spanning tree to be the source of this problem.
When you plug your new 3750,  your spanning tree needs to converge again.

If you dont have loops in your layer 2 topology, maybe what you can try is to enable portfast in the trunk port between your 2 switches.

On stack switch:

int Gig X/Y
 spann portfast trunk

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plus, you must be sure that the new switch will not be the root bridge of the spanning tree domain.

On stack switch:
spanning-tree vlan 1-1000 priority 0


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