What causes undervoltage?

In our small office network, we have about 12 different computers
spread over 4 circuit breakers. Most of the computers are protected by
UPS systems. However, many LAN switches are not.We leave our computers
on 24 hours.
We had a wierd problem late in the night.
1. Two computers reset themselves even though they were on UPS. Both
these computers are on two different circuit breaker.
2. All our main servers are on one circuit breaker and are powered by
a high capacity UPS system. Although these servers did not go down,
the UPS reported a short duration "undervoltage" condition.
One of the resetted computer was on UPS. Therefore, it was not the
electrical power that caused the reset. The only other input to the
computer is the ethernet cable. Here is the layout:
Computer -> Ethernet Cable --> Switch1 --> Switch2 --> Switch3
The computer, Switch1 and Switch3 were powered by UPS. However,
Switch2 was not.
1. What causes undervoltage? Looks like it is not isolated to a single
circuit breaker.
2. Can undervoltage result in a power problem on the ethernet cable
that in turn will reset the computer?
Thank you in advance for your help.
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in general, PCs can be shutdown or restarted by a network admin.
it's conceivable that 'hash' on the LAN could simulate a shutdown command.
the simpler explanation is more likely: the UPS units failed to do their job either by design or by defect. by design i mean the operational window is set to wide on those particular units. on some units the "window" can be changed by performing a series of programming steps. you have to sweet talk the company into providing the programming information.
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Your UPS may have failed - the assumption could mislead you.
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