It's one manifestation of the dreaded Ikea syndrome.
You buy something (what it is doesn't matter) that you have to put together by yourself.
Now, in most of the cases the instructions are either insufficient or otherwise so unclear that they don't help as much as they should. Add to this the fact that in some cases, parts require specific method in order to get them attached to each other. This results in quick frustration, which may lead to slight purple haze in the brains which makes thinking clearly difficult or impossible, which usually manifests as trying to make something the Wrong Way (TM), including the use of Brute Force (TM) to make the Wrong Way (TM) "succeed". When you add alcoholic beverages to the equation, it leads to fully-fletched Ikea syndrome that makes it temporarily impossible to assemble the product in question (properly).
PC parts are no exception to this rule, especially if one is assembling/fixing a computer the first time and doesn't yet know how all works exactly, but the trouble is that PC hardware is a *little* more fragile and less forgiving than your wooden shelf or table you have to assemble yourself. Usually, the wooden or metallic parts don't break easily, and even the use of Brute Force (TM) usually only results in superficial damage.
Computer hardware, however, breaks more easily as a result of Brute Force (TM) - or possibly even as a result of Wrong Way (TM) even if Brute Force (TM) is not needed. And no one pays when you manage to pierce a hole onto your MOBO with a screwdriver when trying to press a plastic latch down or something like that. I almost did that the other day, I mean I also tried to apply Brute Force (TM) to that stupid PCI-E latch, but luckily I found the Right Way* to release the card before any damage was done.
So, when assembling/fixing/upgrading computers it's good to keep in mind that Brute Force (TM) should always be kept far away, and if it seems something can be done in more than one ways, one should find out what is the Right Way before trying the Wrong Way and thus possibly destroying something.
*In the case of PCI-E cards the Right Way to install is as follows (if you didn't have time to figure it out before you mutilated your PCI-E slot

)
-Put the hook in the card's corner (next to the connectors) on into mentioned plastic latch in the slot first
-Tilt the card's rear end towards the MOBO and the slot so that the connector edge slids into the slot and rear panel of the card meets the rear panel of PC
-attach the card's rear panel securely to PC's rear panel.
When removing the card, apply reversed order of installing the card. The hook in the card's front edge comes off the latch the very last.