The Kangxi dictionary lists 塚 as the unorthodox form (俗字) of 冢. However, the usage of 塚 persists in Japan with one stroke removed. In traditional and simplified Chinese 冢 is the orthodox form (正字) of the character. As a variant character, 塚 is used in mainland China.
The /a/ ending may indicate that tsuka developed as the nominalization of the 未然形(mizenkei, “irrealis form”) of the verb, suggesting an original meaning of "that which is being built up into an earthworks (but isn't finished yet)". The irrealis is also the root form for constructing the passive form of all Japanese verbs, so the original meaning might instead have been just the passive sense of "that which is built up into an earthworks".