No, it's just that damage your sentry does increases your crit rate. The same is true for a Medic's patient damage counting for raising the Medic's crit rate.
Could you elaborate on this mechanic, please?
Crit chance is increased based on damage done, it's part of the positive reinforcement mechanic TF2 focuses on. Do well and you'll do better, crit chance is directly related to damage done over the past few seconds, the TF2 blog has done at least one in-depth post on this. One of the core ideas behind TF2 mechanics is that stalemates are bad thus they utilise a lot of positive reinforcement ideas, do well and you will do better, this means a team which is only slightly better than the other will receive additional help to overcome a dug in enemy. If you're on a killing spree your crit chance increases, this helps your team and also helps create those awesome moments when you are on fire and no-one can touch you. Of course the simple fact crits are random crits helps weaker players too, it's no fun to die all the time and random crits help give you an edge over more experienced players occasionally. A
lot of thought went into TF2, it was in development for a lot of time so you wont find any area that hasn't had a large opart of the team debating and focusing on for a large amount of time.
I agree with Greiger, crits have had a lot of thought put into them, the only time TF2 shouldn't have them is during professional tournaments when random chance cant be a deciding factor. Unfortunately people remember the times they die to crits not the times a crit helped them take out half the enemy team or gain a revenge kill on that enemy that has dominated them the past 2 games.