Judging from what you've written, I'd wager you're electing to grab, your combat prefs are set to close combat, you're directionally moving which triggers a grab auto-attack, which is why you grab randomly. When you've switched to charge, that directional move is making you charge. The best combat pref setting here would be strike, as strike's auto-attack won't cause a charge which may move you one tile, or randomly entangle you if set to close combat.
So when you press (shift + A) to open the attack menu, followed by choosing selection (b) to wrestle, followed by whatever you're choosing to grab, your choice here is either to process the grab, which confirms that it's landed/missed/blocked/they jump away, or you can chain in another action, which means you don't wait for grab confirmation.
If you want to confirm that the grab has landed, you press the comma key (,) which is one tick, as opposed to the period key (.) which is 10 ticks. The only reason to wait to process the grab is when you want to proceed further down the grappling road map to either a joint lock or a throw. If you choose to wait 10 ticks, all kinds unfortunate things may happen during that interval.
Since it seems you want to disarm, this means you want to chain in another action. You'd want to (shift + A), then (b) to wrestle, choosing to grab the weapon, followed by either (j)umping away to perform Kisat Dur's "grab jump", or you could again (shift + A), followed by (b) to dodge, in order to perform Kisat Dur's "grab dodge" to perform the disarm.
A thing to bear in mind is your character's success in maintaining control over the weapon grabbed during "grab jump" and "grab dodge" is linked to your character's size and strength. An average size/strength goblin will fail to disarm an average size/strength human.