If deacons are servants, and they are, why do they exercise power and influence in their congregations?
Perhaps it is that, ideally, the deacons are the wisest and most spiritual men in the congregation. The congregation looks to them and gives weight to their opinions. The congregation delegates to the deacons certain responsibilities, and the authority commensurate with that responsibility.
It's not hard to see how such authority could be abused, or even assumed to go with ordination. Even the congregaton may assume it, wrongly.
When the congregation mistakenly elects unspiritual men as deacons, trouble is not far down the road.
Oh, by the way, it can happen under the elder system as well.