How to write a sentence that explains why the previous sentence is true?

I often have to present a complex result and explain why it is true. Doing so in a single sentence usually makes it too long and difficult to understand. But how do you succinctly do this in two sentences. For example consider the sentence

When stocks increase in price, people want to buy them, increasing the price further, until the stock reaches a critical threshold, after which people sell and the price falls. The reason why this is true is that ...

I'm looking for some less clunky starting phrases, to start the second sentence, that warn the reader that an explanation is about to follow and not a new idea. Or I am also looking for other ways to deal with this issue.

This is because...

They do this because...

; (replace the period with a semicolon)

The most economical means of signaling that a sentence provides grounds or warrant for the claim advanced by the preceding sentence is to begin the second sentence with For. OED s.v. for, B. conj., 2.a.:

Introducing the ground or reason for something previously said: Seeing that, since. Cf. Greek γάρ, Latin nam or enim, French car, German denn.

Note that the English equivalents listed here (“seeing that, since”) cannot so well lead off a separate sentence, though they can be used to introduce grounds or warrant for a preceding clause where only a comma precedes.