"When we ping to any server then we see pattern drop so why did we see like this?"
There are multiple possible reasons, not all exclusive with each other.
Some of the more common reasons include:
Host "thinks" it might be being "attacked" with successive ping requests, so it throttles responses. (As mentioned by @MHM Cisco World.)
Host is particularly "busy" and doesn't want to waste its resources on replying to every ping request. (Same reason might also cause highly variable ping response times. I.e. host delays its response to when it's not as busy.) (Sort of also like WC Field's "go away kid, you bother me".)
Ping requests (or corresponding ping responses) are dropped, in transit, due to congestion somewhere along one or both (i.e. to and/or from) paths.
BTW, ping, fundamentally, is really to allow one host to determine if another host is "alive". So, not responding to every ping request, from the same host, is fine for that purpose.
