Why does in some cases the verb in German come at the end of the sentence?

Normally, just like in English, the German word order is Subject - Verb - Object (e.g. Ich werfe den Ball - I throw the ball). If you use a coordinating conjunction, such as 'und', 'aber', 'denn' or 'oder', then there is no effect on the word order (e.g. Ich renne vorwärts und ich werfe den Ball - I run forward and I throw the ball). However, if you use a subordinating conjunction, as 'während', 'bis', 'als', 'wenn' or 'da', the first verb in the clause moves to the end of the sentence. In practice, consider this example: Ich kann ihn nicht leiden, weil er mich immer auslacht. (I don't like him, because he always makes fun of me.). As the subordinating conjunction 'weil' is employed in this sentence, the 'auslacht'  has to be placed at the end of the sentence.

Similarly, if you use modal verbs, which are 'helping verbs' that cannot stand by their own (e.g. sollen, müssen, want, können), the second verb in the sentence is always in the infinitive and comes at the end of the sentence. The infinitive is easy to spot, as most German verbs end in '-en'. If it is a question the modal verb is additionally at the beginning of the sentence. 
Example: Ihr solltet mir mit meinem Umzug helfen. (You should help me with my move.)