Learn Finnish: Ottelijat kumartavat toinen toiselleen.

Both mean "to each other," but the double-pronoun structure toinen toiselleen (literally "one to its other") is a more formal, distributive way to express reciprocity. In this structure, the first toinen is always left in the nominative singular, while the second takes the grammatical case required by the sentence.

Why is the second half in the allative case (toiselleen)?

The verb kumartaa (to bow) requires the target of the bow to be in the allative case (the -lle ending), answering the question "to whom?".

Why does toiselleen have the suffix -en attached to it?

In Finnish reciprocal phrases, the inflected pronoun must take a possessive suffix that matches the subject. Because the subject ottelijat is third-person ("they"), the third-person possessive suffix -en (or alternatively -nsa/-nsä) is added to the allative case ending.

Since ottelijat is plural, why isn't the first pronoun pluralized as toiset toiselleen?

The distributive structure almost always uses the singular toinen for both parts when individuals are interacting with other individuals. You would only use the plural toiset toisilleen if multiple groups of people were interacting with other groups.