Learn Finnish: Miksi piirsit punaisen viivan? - Why did you draw a red line?

They are the object of the sentence. Because drawing the line is a completed action, Finnish uses the accusative case, which looks exactly like the genitive -n ending for singular words. If the action was ongoing or incomplete (Why were you drawing a red line?), you would use the partitive case: punaista viivaa.

How do you get piirsit from the dictionary form piirtää?

To form the past tense, you add the past marker -i- to the verb stem. In Finnish, a t followed by the past tense i often changes into an s. So the stem piirtä- drops its final vowel, the t becomes s to make piirsi-, and finally you add the -t ending for the 'you' form.

Why doesn't the verb have consonant gradation here, like it does in the present tense form piirrät?

The past tense stem here is piirsi-, and the consonant combination rs does not undergo consonant gradation in Finnish. In the present tense, the stem is piirtä-, and the strong rt weakens to rr because the personal ending -t closes the syllable.