Unnecessary raise of rpm

Last Saturday evening I took my x440t non stop for 52km when I was almost nearby to my destination i have observed that the rpm was raising for no reason. Immediately I have pull over to a place switch off the engine and later after 5 mins started again but still the rpm kept raising which resulted in milleage drop and it kept raising until my destination and in the next morning when I have started the bike it is normal again. Could anyone drop the reasons for unnecessary raise of rpm I cannot get it till now.