Learn Finnish: Miksi ovi rikottiin? - Why was the door broken?

The -i- in the ending tells us it happened in the past. The present passive for "is broken" is rikotaan, but to make it the past passive ("was broken"), the ending changes to -ttiin.

How do we form the rest of rikottiin from the basic dictionary verb rikkoa?

Rikkoa (to break) is a verb type 1. To form the passive, we take its weak stem, riko-. Notice how the strong double kk weakens to a single k because of consonant gradation. Then we just add the past passive ending -ttiin.

Why is the object ovi in its basic dictionary form instead of oven?

Normally, a complete object takes the -n ending (like oven). However, a major rule in Finnish is that when the verb is in the passive voice, the object drops the -n and is written in the basic nominative form (ovi).

Does this sentence mean the door just broke on its own?

No, the Finnish passive voice always implies that an unspecified person did the action. It strictly means "was broken by someone." If you wanted to say the door broke on its own, you would use a different phrase entirely, such as ovi meni rikki.