until now I am still curious
A bubble is an asset whose price rises faster than the value proposition should dictate. Since bitcoin is an asset with no inherent value, the value ostensibly comes from one of two places: 1) utility to perform a useful function (value transfer or store of value) or 2) speculation that someone in the future will be willing to buy the asset off of you for more than what you paid for (the later idiot fallacy). Bitcoin was supposedly created to fulfill number 1, value transfer/store of value, but it is much MUCH more widely used for number 2, which makes it prone to inflated price due to hype, FOMO and the collective delusion that the price cannot go down for [insert any number of unsound reasons].
Because there is no reliable mechanism for shorting bitcoin (an equilibrium force in the market), price is pressure primarily builds in one direction without a reliable counter force (someone betting against the price by shorting), and this further tends to skew assets like this into bubble territory. The bubble pops much more violently in these cases than assets that can be shorted easily because the buying pressure suddenly disappears and then there is a rush to get out of the asset by dumping coins on a smaller pool of buyers.