critical swap usage alert of Host

I am getting critical swap usage alert in one of the monitoring tool called nagios.

1) However when I went to the host from vCenter and checked the memory utilization from the cluster, the respective host memory utilization is only 66%

Also I went to performance tab of a host, when i selected memory, it shows a 1 min high spike and suddenly it went down.

My question is how do I identify, which VM is using more swap on this Host or what is the cause of  that high spike?

2) From ESXi command line, esxtop "m" output shows the below

SWAP  /MB:   123    curr,   122 rclmtgt:                 0.00 r/s,   0.00 w/s

which means it is using 123 MB swap and reclaim memory is 0.

My query is, when I can see it is showing 66% utilization of Memory, then why it is using swap? and I am not sure, how do I avoid to such situation in future?

Can anybody please, I have searched and didn't found anything relevant.