[CentOS] Automatical resync of MD, why?

Hello,

I have three MDs on two HDDs in RAID 1. In last days, thanks to my
monitoring (which is installed longer time), I have recognized, that
one of these MDs is automatically started resync. First was at 25th
October and second resync of the same MD was today. It started in same
daytime (about 4:00 AM). Now is MD resynced and working well. Can
someone tell me what means this automatic resync? Is it standard? Is
it means some possible problems with one HDD (but other two MDs are
working without resync)?

Thank you
Martin Šťastný