Rampart can now melee her opponents ridiculously quickly because of an exploit made possible after recent buffs to her ultimate ability, Sheila. The rapid melee glitch is somewhat of a notorious one for Apex Legends at this point because it has been discovered and patched out multiple times before.
The exploit works by interrupting the end of the melee animation with Sheila's equip animation. As soon as your melee attack connects with an opponent pull out Sheila. Repeat this two-step process as quickly as you can and let the melee lunge help you connect with the enemy. If done correctly, players will be able to melee opponents at roughly double the normal speed.
Pathfinder and Horizon both used to be able to use the same exploit in earlier seasons. Pathfinder was able to cancel his grapple animation and Horizon was able to cancel her singularity's equip animation.
The glitch is pretty frustrating to come up against because of how difficult Apex Legends' melee lunge is to escape and how much damage melee attacks do, especially so quickly. Fortunately, Respawn has patched this kind of exploit out before and is likely working on a fix for a coming patch.