'डेढ़ बजे' is in the oblique case. This is when a noun is the object of a postposition. Though there is no postposition in this sentence, को is implicit (corresponding to 'at' in the English sentence). वह डेढ़ बजे (को) आती है.
The oblique form of बजा is बजे (which is the same as its plural form here). Since times are almost always used in the oblique case, you will see बजे much more than बजा except in sentences such as 'अब डेढ़ बजा है'.
It comes from the past tense form of बजना (to ring/chime) Eg: घड़ी दो बार बजा - 'The clock chimed twice' from the era of grandfather clocks eventually becoming 'घड़ी में दो बजा'(It rang two in the clock) in turn giving rise to the usage 'अब दो बजे है' (It's two o'clock).
At present, it is used as part of a noun phrase (number followed by बजा(sing)/बजे(plural)) like in the last sentence above or 'वह डेढ़ बजे आती है' without really acknowledging its roots. This is similar to how 'o'clock' is now a word in its own right in English though it comes from 'of the clock'.