I've always wondered this, and I've never found an answer. Seems a bit of an odd thing to do but there must have been a reason behind it, right?
They didn’t unload their weapons before “going over the top”. Why would you believe they did? A military rifle such as the .303 Lee-Enfield has the following degrees of readiness:
Unloaded – No round in the chamber and no rounds in the magazine (or magazine removed),
Loaded – No round in the chamber but rounds in the magazine,
Action – A round in the chamber, rounds in the magazine and safety catch at Safe.
Instant – A round in the chamber, rounds in the magazine, safety catch at Fire and prepared to fire.
On a bolt action weapon such as the .303 Lee-Enfield to chamber a round requires the soldier to work the bolt by pulling the handle to the rear and pushing it forward again. The same action is required to remove (eject) either a fired or an unfired round from the chamber and is part of the action required to unload a weapon.
If your belief stems from watching movies such as Gallipoli, it is possible that you have mistaken the action of working the bolt to be unloading the weapon rather than chambering a round, (changing from the Loaded condition to the Action condition). In a movie this action would be for dramatic effect, as soldiers at the front line would almost certainly already have the weapon at the Action condition and in preparation for “going over the top” would simply change to the “Instant” condition by shifting the safety catch.
Yes, there is a scene in the movie, just prior to an attack where an officer walks down the line saying approximately, "Remember men, nothing up the spout, we are going in with the bayonet". This means that their rifles are to be in the Loaded condition with no round in the chamber.
The movie is not all that historically accurate but this particular scene is consistent with the orders given to the Light Horse (L.H.) for the attack on the Nek, Baby 700, Pope's and Quinn's Posts on 7th August 1915. The initial attacks were to be with bayonet and bomb (grenade) with no firing. Having weapons in the loaded condition would prevent accidental discharges.
The orders were as follows:
“8th L H. 1st Line.-First line will consist of troops already in fire-trenches and saps On a given signal, silently and without rifle-fire, it will rush The Nek (Ai) and with bayonet and bomb engage the enemy, taking possession of the flank, communicating and advanced trenches (A9, As. A8, AI]), paying special attention to the machine-guns. which must be sought for and rushed. and to the trenches overlooking the cliff north of The Nek and to those on the southern flank of same. so as to prevent flank interposition by the enemy-mine fuses and ‘phone wires to be sought for and cut.
8th L.H. 2nd Line -Second line (already on banquette) will immediately follow. Jumping advanced trenches (already engaged by first line) it will sweep on and attack supporting and subsidiary trenches (AIa, CI, C4). Its action will be forward, ignoring trenches behind, but accounting for those to right and left (C64 BI, Bz. B3). Bayonet and bomb without fire
Note.-As soon as first line has moved from our trenches. second line will take the position vacated in order to make room for third line. In passing over intervening space officers will take post in the ranks so as not to make themselves a conspicuous target
l0th L.H. 3rd Line.-Having moved up communicating trenches, third line will in like manner he prepared and follow on at once. Its objective will be the next line of trenches (Ca, C3, C5, C7, C8) and, if possible. Z. Y, CIO, CII, to CIZ-13. With bomb and bayonet only. the enemy will be driven back and out without turning back, and avenues blocked. Once in the trenches, the enemy will not be able to-make effective use of his machine-guns. When the extreme limit of advance has been reached the gain must be made good and safe against machinegun fire and against counter-attack. Here fourth line plays its part.
10th L.H. 4th Line.-Fourth line will in like manner follow and act in concert with a and 3. Every second man will carry digging tools in the proportion of one pick to two shovels. It is impossible to define precisely what this line may be called upon to do. This must of necessity depend upon the progress of its predecessors. It may have to down tools and assist. but it must make every effort to join up with third line and block the approaches. It must endeavour to join up with the latter. This is its role.>
C.E.W. Bean: Official History of Australia in the War of 1914-1918, Volume 2, Chapter XXI The Feints of August 7th