You've got a keen eye!

The reason lies with Japanese anti-tank tactics. After the New Georgia campaign in 1943, there was a need to protect tanks from magnetic anti-tank mines (Magnetic Antitank Mine Type 97) after the IJA made extensive use of them. Due to a lack of a hand held anti-tank weapon like a Bazooka or a Panzerfaust, the Japanese resorted to (amongst other things) satchel charges, Molotov cocktails, grenade clusters, magnetic anti-tank mines or simply attaching an antitank mine on a pole and then just running up to the tank and thrusting the pole onto it. To combat the use of magnetic anti-tank mine, the development and application of wooden (and/or concrete) appliqué side armor was carried out. Even extended side armor in the form of wooden skirts were placed over the suspension bogies on some tanks to protect from satchel charges. Despite of this, it wasn't universal due to supply problems and there were plenty of tanks who either used alternative material for protection (like the previously mentioned concrete or sheet metal) or were simply just knocked out by magnetic mines well towards the two last year of the war.

Source:

U.S. Marine Corps Tanks of World War II by Steve J. Zaloga.

World War II Infantry Anti-Tank Tactics by Gordon L. Rottman.

Further to what's below, the shaped charge emerged during WWII and was used by all combatants. By hollowing out the face of a piece of explosive, the detonation focuses the explosion onto a point, like a magnifying glass can turn sunlight into a hot spot. This could melt armour, and did not rely on velocity. So you could fire it from a light weapon, or carry it as a mine. It was the basis of the bazooka, panzerfaust, and many similar weapons. But, the bomb had to be held at an optimal distance from the armour for the focus effect to work, hence the mines attached by magnets or, better (cheaper) still, something sticky.

To the Japanese, the cheapest commodity of all was the soldier, so they eschewed expensive rockets and magnets and developed what the Allies called the 'lunge mine'. Ian Hogg describes it thus:

This was a powerful hollow charge unit mounted on the end of a bamboo pole and fitted with an instantaneous fuse operated by a string tied to the pole. On the approach of an Allied tank the Japanese volunteer would leap from hiding, holding the lunge mine up in front of him, rush up and jam the explosive charge against the tank. He then said a quick prayer and pulled the string. The resultant explosion pierced the tank with the hollow change- and the soldier with the bamboo pole" [Land Power, Exeter Publishing].

Being burnt to death inside a tank is no fun, and tankers were more or less as keen to survive the war as anybody, so they developed all sorts of desperate measures to enhance their chances against the hollow charge, and spacing the explosion away from the main armour was (and remains) a key method.

Soldiers would use whatever they had, and this was often lumber and concrete. Note in your pictures there are other typical Pacific War modifications, solid road wheels, hatch suspension protection. The ill-equipped Japanese would immobilise tanks by forcing logs and iron spikes into the suspension units. In the bushy country on Pacific Islands tanks could easily be rushed if nearby infantry put their heads down.

In the ETO additional armour became common in late 1944 as the fighting became 'short range' in villages and the wooded areas of Belgium and so on. The logs and so on give no extra protection against high-velocity anti-tank weapons used in the open fields of France (in Normandy anything stuck to the side of a tank was pulled off driving through hedges, and there were still few casualties from shaped charges thanks to close infantry support). Commanders of the 'dashing' armoured divisions in Europe often discouraged adding stuff to tanks in this way, deeming it unsightly, defeatist and adding weight (Patton famously hated such add-ons). In the Pacific, every tank was an infantry support weapons, and with long waits in depots and so on before a landing, there was amply opportunity to modify tanks.