what does this log mean ? why ?

Major 192.168.XXX.XXX SW_XXXX_0healthMonModuleTrapSlot 2 module-level, Rx: noChange, RxTx: noChange,Memonry noChange, CPU crossedBelowThreshold,Chassis ID noChangeNov 13, 2019 2:12:29 pm

I guess this log is coming from the NW management system right. In that case, you should have used the category "OV 2500 NMS" when posting your question to go directly to the right forum area.
This "Help" section is intended to get answers on the Spacewalkers community site rules/use/how to for members.
BTW, I have updated this for you.
Thanks for being part of this community and do not hesitate to exchange with your peers on any area to get or provide support. Regards -