I imagine that these symbols originated in one of the following ways:
"I will declare a symbol for "for all." I will just use the letter "A" flipped upside-down. Yes, let $\forall$ represent "for all."
Hmmm, now I need a similar notation for "there exists." Well, I will do the same thing with the letter "E". But wait! The letter "E" has horizontal symmetry, so this won't work! I must flip it backwards instead. $\exists$ it is!"
OR
"I will declare a symbol for "there exists." I will just use the letter "E" flipped backwards. Yes, let $\exists$ represent "there exists."
Hmmm, now I need a similar notation for "for all." Well, I will do the same thing with the letter "A". But wait! The letter "A" has vertical symmetry, so this won't work! I must flip it upside-down instead. $\forall$ it is!"
So which is it? Did we flip our A's or our E's first?
The $\forall$ (for all, universal quantifier) symbol first appeared in the 1935 publication Untersuchungen ueber das logische Schliessen ("Investigations on Logical Reasoning") by Gerhard Gentzen.
The $\exists$ (there exists, existential quantifier) symbol was first used in the 1897 book Formulaire de mathematiqus by Giuseppe Peano.
$\exists$ came first.
(source)
Untersuchungen euber das logische Schliessen page 178 appears to be the first use of the symbol $\forall$ by Gentzen in a publication.
A googletranslate of the footnote at the bottom reads: