Cursor selection goes wild?

It is really weird that now (after upgrading to Yosemite) I have great problems controlling my text selection.

For example, after selecting a word, I press command + c to copy it. Once command is pressed, the cursor wildly goes forward, selecting a lot of other words. It is really annoying.

Another similar problem is that once I select a word and press delete, the cursor also goes wildly forward, selecting a lot of other words and deleting them all.

Why is it so, and how may I fix it?