The RTS serial port signal is used to shut down the UPS. The UPS will
shut down only if it operates from its battery. The manual says that
the shutdown signal must be high for at least 0.5s. But few milliseconds
is enough, at least for my APC Back-UPS 600.

Using RTS to shut down the UPS can be dangerous, because the RTS goes
high when the serial device is opened. The backupsd program then turns
RTS off, but it is on (high) for a moment. This kills the power when
backupsd is first started and there is a power failure at this time.
This can happen for example when the UPS is shut down, unattended,
and the power comes back for a while.

Either start backupsd before mounting any filesystems for read-write,
or (better) use TX (pin 3) instead of RTS (pin 7) to shut down the
UPS (pin numbers are for 9-pin plug). Use ioctl(fd, TCSBRKP, 10);
to make TX high for one second, for example. Using TX should be safe.
Maybe I will post the diffs if time permits...