Why do some words have two past tense forms (e. g. "dreamed" vs. "dreamt") ?

This is essentially language change at work. Do you know how Shakespeare sometimes sounds "weird" to us? that's because English has changed since then.

Language change is not abrupt, but gradual, and while the change takes place sometimes two forms are used: so while it is likely that in years to come only dreamed will be correct, and in the past only "dreamt", currently the change is not complete and both are correct.