Why do planes encounter strong convective weather when flying in the stratosphere? Aren't these only found in the troposphere?

The stratosphere is only a relatively stable layer of atmospheric movement, and its height is above 10000M, but the violent atmospheric movement in the troposphere spreads to the stratosphere. For example, the height of strong convective cumulonimbus clouds can reach 27,000, so even in the stratosphere, there will be occasional strong convective weather, but this situation is rare.

Another situation is when the plane crosses two floors in the process of ascending or descending.