why do errors stop during backup

[fxkl47BF]

i have an old pci analog video capture card that uses bttv
it fills the log with timeout errors, 1 or 2 per minute
bttv: 2: timeout: drop=725178 irq=4905870/4927747, risc=2014e424, bits: HSYNC
each night i run a backup with rsync
during that backup time there are no bttv errors
the cameras continue to capture normally
i realize the details of this are probably way over my head
can anyone explain this in simple terms