MHT is the duration during which a MS is using the TCH in the cell.
So if it is too short, it means either the calls are dropped as soon as the TCH is seize… or there is a HO taking place immediately. Check your drops and your outgoing HO : are they high ?
If drops : HW problem, probably
If HO : settings are done in such a way that HO are triggered quickly from this cell. It could be a correct strategy (for example, if cell is an umbrella : the calls are sent to microcells immediately).