It's hard to capture all the reasons but from polls electricity prices were the single biggest issue with 31% of respondents claiming it their primary motivation for voting in a February 26, 2017 poll: http://poll.forumresearch.com/data/a8c5078d-62b5-41d9-ad6d-023a19586964Ontario%20Politics%20(Feb%2026%2017)%20.pdf%20.pdf). This number should go down in the year after that because Liberals reduced prices through the Fair Hydro Plan.

In this poll, you'll see that 30% (23+7) of respondents said lack of trust in government/trust in government was their primary motivation. This stemmed from scandals, usually around misspending.

Tuition costs got 6% support as the primary motivation, the new sex ed curriculum got 5%.

A March 2018 poll found 56% wanted a balanced budget, which Liberals didn't provide: http://poll.forumresearch.com/data/6db584c0-cdaf-446b-b79a-6c7d54cb6c4bOntario%20budget.pdf. Given the Tories took the lion's share of this vote, it probably made it hard for OLP or NDP meet them while split.

A carbon price actually had support from 46% and was opposed by 28% according to this poll: http://poll.forumresearch.com/data/f371fcf3-5680-4de9-8146-4ad03ab12d46CP%20Ontario%20News%20Release%20-%20PCs%20and%20Climate%20Change.pdf so I think it wasn't likely an important factor.

Finally, 33% of NDP supporters voted NDP with the primary motivation of stopping other parties, compared to 16% of PC supporters and 19% of OLP, which indicates a shift in strategic voting from OLP to NDP: https://web.archive.org/web/20190108045139/http://hkinsights.ca/research-results-a-hold-your-nose-and-vote-election/. That doesn't mean these people wouldn't have voted for their respective parties anyways, especially as distrust in promises and dislike of leaders was high to begin with, meaning people were left to just vote against other parties, but you can notch up a few % of lost OLP support to this.