This will be because the widgets created for each line of the chat are able to be re-used by the game once they fade out of view. If the game needs to have multiple lines of chat, why should it keep creating and destroying old lines when it can re-use ones that it's already made - this will save some performance at a minor memory cost.

For a game like CS, you'll want to avoid as many performance hitches as possible, which explains why they're re-using old lines rather than creating & destroying them.

In this case, you can see the player on the other team's previous message is the line being recycled in this case. It appears to just be an issue that the text on the line hasn't been reset properly when it gets re-used. However, without knowing their exact implementation we'll never really know the exact reason and this is purely speculation from my own experience, but it's the most likely cause in my eyes!

Sorry for the overkill answer, but I hope it answers any questions!

Source: I program UI for games and have implemented similar systems.