Why does one "jump" bail?

Solution 1:

I'm not sure exactly where this usage of jump came from, but in general it is common to use this verb to indicate figuratively some sort of sudden, illicit movement. The jump in to jump bail is definitely of this form, as in the individual has 'jumped' free, or out of, their bond.

Your example is similar to the phrase to jump ship, which was originally used to refer to sailors who abandoned their position on a ship while it was in port, and is now a common idiom for abandoning some kind of role or duty.

Other examples of the verb to jump being used figuratively to indicate something criminal include

• to jump the queue/line = to push in ahead of your turn (OK not exactly criminal, unless you come from the UK!)
• to jump someone = to attack someone

Solution 2:

Jump bail

• To default on one's bail (1872+ Underworld)

(The Dictionary of American Slang, Fourth Edition)

also Skip bail:

• Or jump bail . Fail to appear in court for trial and thereby give up the bail bond (paid to secure one's appearance). For example, I can't afford to skip bail—I'd lose half a million , or We were sure he'd jump bail but he finally showed up . This idiom uses skip and jump in the sense of “evade”. The first dates from about 1900, the variant from the mid-1800s.

The expression is from criminal slang as suggested above. According to Criminal Slang: The Vernacular of the Underworld Lingo the verb jump could refer to "an illegal seizure", from which probably the usage of "jump bail".

Solution 3:

On a few occasions, verbs of movement do take a direct object (or what syntactically appears to be a direct object) which however functions as an ablative: flee the country, fly the coop, jump ship, etc. Flee the country means "escape from the country_". Analogously, jump the bail means "escape from (the state of being on) bail".

I believe this is an interesting phenomenon (and have not bee able to unearth any discussion thereof), but one of syntax, rather than etymology.